I'm not qualified to dispense medical advice; what follows is merely the product of my personal experience. For a much more comprehensive and authoritative view of medical issues as they pertain to traveling, refer to Dr. Stephen Bezruchka's excellent (and wonderfully portable) The Pocket Doctor.
toilet paper, antibacterial wipes You can't depend on finding toilet paper everywhere. You can't even depend on finding recognizable toilets everywhere. The minute you leave "western" civilization, you will quickly discover that considerably more than half the world uses some variant of the squat toilet. You will find modern versions of these in Japan, and trenches in China (when on safari in some desert, you likely won't find toilets at all). If you're unfamiliar with the use of squat toilets, your first encounter can be a daunting one indeed! Advance practice is highly recommended.

Briefly, pants are rolled up to the knees, and the upper part, along with underpants, lowered to the knees (you don't want them around your ankles!). Items capable of falling are best removed beforehand; retrieving them will likely be impractical. You squat by first bending the upper part of your body forward (to maintain balance), then lower yourself by bending your legs, coming to rest quite naturally (and comfortably) on your haunches and legs. Reverse to rise. Aim is more important in squat toilets than in sit toilets, so don't fire indiscriminately.

Cleaning up is likely not to involve toilet paper, unless you brought your own. This is done using a (probably nearby) container to bring water to your waiting left hand, which will splash it on the appropriate areas. Washing one's hands when finished takes on a new importance; locals may well do this with water only, so you might consider bringing your own soap. Alternatives to soap include hand wipes treated with antibacterial solution (ensure that "Ethyl Alcohol 62%" is one of the active ingredients; one good brand is Purell Sanitizing Hand Towels with Moisturizers), and liquid antibacterial cleansers (often available in travel-sized containers), though the latter introduce the challenge of transporting liquids. There is some dispute among experts about the long-term value of antibacterial products like these (and none of them work on viruses anyway); all agree, however, that frequent washing is more important than the choice of ingredient to abet same. Unfortunately, most people wash their hands in a perfunctory fashion that accomplishes little. It should be done, with soap or the equivalent (which suspends the microbes such that they can be washed away), for at least 15-20 seconds, using warm (or cool, but not hot) water, and should include the backs of the hands, the wrists, and under the fingernails.

And now you know why the social use of the left hand is impolite in many parts of the world (it's also why punishment in some countries involves cutting off the left hand). If you do use paper, do not deposit it in the toilet (unless there's no plumbing involved), or you will probably clog the plumbing. There is likely to be a wastebasket handy: the soiled paper goes there. Yes, really.

There's much more to this topic than mentioned here, as there is truly an amazing variety of toilets to be found in the world. Read "Going abroad" if you really want to expand your horizons.
(Chemical or mechanical) water purifier If you're traveling anyplace where sanitation is even the least bit questionable, this is a must. The problem, and its several solutions, is too diverse to cover here, but you really should learn about this topic: Rob Sangster's book is a reasonable (and current) reference, and Travel Medicine, Inc. sells a variety of purifiers. Bottled water can be expensive, and not always available (or even trustworthy; in this respect, bottled carbonated water is safer). Chemical additives taste bad, and may have long-term health implications. Mechanical (filter) purifiers are bulky, heavy, and most don't remove viruses.

Boiling water is the best method for eliminating infectious agents. There is some debate about how long to do this, but sustaining a strong boil for at least a minute generally renders it safe to drink.

The next best choice is probably the venerable 2-3 drops of iodine in a liter (quart) of water, left to stand for 30 minutes. Yes, there's a bit of a chemical taste, but there is with chlorine-based additives as well, and they don't kill giardia. You can pretty much remove both the taste and the colour by adding 50 mg of vitamin C (ascorbic acid) to the water, after it has stood for the treatment period; a flavored beverage mix containing vitamin C is fine for this. Be aware that iodine stains terribly, and runs like crazy to boot, so pack it accordingly; if your iodine container ever leaks inside your bag, you'll never forget it! You can find de-coloured iodine in the U.S. and some other places, and the regular stuff at pretty much any pharmacy in the known world.

Popular wisdom states that iodine should not be ingested on a long-term basis (more than six months), but I have never seen any reputable data that confirms this. On the contrary, a study of the previously healthy inmates of three Florida prisons who consumed water disinfected with 0.5 to 1.0 PPM iodine for 15 years showed no effects on health or thyroid function. Of 101 infants born to prisoners drinking the water for 122-270 days, none showed detectable thyroid enlargement. Nonetheless, those with thyroid problems might consider alternative solutions.

Two final comments: avoid ice (which cannot be assumed sanitary) in questionable environments, and remember to use safe water when brushing your teeth!
Analgesic of choice Aspirin, acetaminophen [paracetamol], ibuprofen, naproxen sodium, ketoprofen, or whatever, are best (though not most efficiently) carried in their original packaging, especially if you look somewhat "counterculture", and/or expect to meet suspicious border inspectors.
diarrhea treatment (e.g., Lomotil and/or Cipro) Studies show that 35% of all international travelers (and 50% of those visiting developing countries in Latin America, Africa, the Middle East, and Asia) develop "traveller's diarrhea" (TD). So there's a pretty good chance that this will be a part of your experience at some point, making it helpful to know something about its causes, prevention, and treatment. Several infectious agents, including bacteria, parasites, and viruses, can cause TD. While parasitic TD can occur 7-14 days after exposure, most shows up some time from a few hours to three days. In up to half of all cases, the exact cause is unknown. The most common bacterial cause of TD (20-50% of cases) is Escherichia coli (E. coli), found in soil, water, and milk. Diarrhea caused by E. coli is generally explosive, non-bloody, and accompanied by nausea, vomiting, cramps, and fever. Shigella is the next most frequent (15% of travellers) bacterial cause of dysentery, being common in countries experiencing natural disasters, socioeconomic upheaval, or other conditions making it difficult to procure sanitary food and water. Shigella infections produce bloody and mucus-laden diarrhea, along with fever, cramps, and exhaustion; such severe consequences are usually labelled dysentery. Next in line (2% of cases) is TD caused by the parasites Giardia and Entamoeba histolytica, found in contaminated water. Giardia can cause chronic diarrhea lasting for several weeks, along with vague pain, weight loss, bloating, and fatigue. And there are other bacteria (Salmonella, Campylobacter), parasites (Cryptosporidiosis, Cyclospora), and (less commonly) intestinal viruses like rotavirus. As you might imagine, dealing effectively with all these possibilities is not an easy matter.

Prevention should be the first line of defense. Don't ingest (or swim in) contaminated water; see the above discussion of this topic. With respect to food, the traditional traveller's adage ("If you can't cook it, boil it, or peel it, then forget it!") is still valid, but doesn't go far enough (in the light of current research). Heated food should be hot to the touch, and eaten promptly. Be particularly cautious of food from street vendors. Peel your own fresh fruits and vegetables (be especially wary of things like lettuce and sliced fruit, which may have been washed in contaminated water). Avoid dairy products and raw or undercooked meat and fish.

Pepto-Bismol (two tablets four times daily before and during international travel, but not for more than three weeks) can help prevent many cases of TD. In common with aspirin (which shares its active ingredient salicylate), its use should not be indiscriminate: many medications interfere with salicylate; pregnant women, people allergic to aspirin, and those with ulcers, other bleeding disorders, or gout, should first consult a physician; children under three, and children and adolescents with flu or chicken pox, should not take it. Possible side effects of Pepto-Bismol include ringing in the ears, and black stools and tongue.

The two most popular prescription drugs for dealing with TD are Lomotil [diphenoxylate atrophine], which essentially blocks your plumbing for a day or two, and Cipro [ciprofloxacin hydrochloride], an antibiotic to kill the bugs that are causing the problem. As with any prescription drug, discuss these with your doctor before taking them. The indiscriminate use of antibiotics is causing a lot of problems in the world.

Be aware that antimotility drugs such as Lomotil (and Imodium [loperamide], an over-the-counter alternative that works better for some people) do not prevent diarrhea (in fact, they may even increase the risk for it), but they provide prompt though temporary symptomatic relief by reducing muscle spasms in the gastrointestinal tract. Don't use them unless access to a toilet is limited (such as during a long bus ride). And never use them for more than 48 hours.

As a rule, letting nature take its course is the preferred approach, abetted with plenty of fluid replacement (drink lots of pure water; potassium, sodium, and calcium supplementation is also advisable). Be aware that rice, wheat, potatoes, corn, and chicken are foods known to slow the effects of TD. And understand that the prevention of dehydration is the most important consideration: in severe cases, dehydration can be life threatening, particularly in children. Agitation may be an early symptom; severe indications include listlessness and a weak pulse, and are causes for serious concern.

Infection treatment (e.g., Keflex) another prescription drug, Keflex [cephalexin] is one of the cephalosporin antibiotics. It can be used for a wide variety of bacterial infections, including those of the respiratory tract, urinary tract, skin, and ear. Again, consult your doctor for details.
Malaria tablets a must in many parts of the world.

Insect repellent, mosquito net Even if you're not trekking in the bush, a lightweight, over-the-head mosquito net can save you from hotels whose ideas of adequate screening don't coincide with your own. In many areas of the world, you'll want a net (such as those available from Long Road Travel Supplies) large enough to cover your entire sleeping area. Spraying a net (and your clothing, for that matter) with permethrin will increase its bug repelling effectiveness. For your skin, you need something containing either deet [n,n-diethyl-3-methylbenzamide] or picaridin [(RS)-sec-butyl 2-(2-hydroxyethyl)piperidine-1-carboxylate]; the higher the concentration, the more effective. Much enthusiastic advertising to the contrary, these are the only insect repellents that have proven effective in controlled, objective tests.

Picaridin (also called KBR3023, Bayrepel, Hepidanin, and Autan Repel) is the most recent development in this field, and is considered less irritating to the skin than the time-honoured deet, to which it provides comparable protection. Both products are considered safe to use, despite the occasional rumour to the contrary, suggesting that deet is scary stuff (numerous credible tests have confirmed that deet is safe when used correctly, even for children and pregnant/lactating women; some people do, however, find that it irritates their skin, especially eyes and mucous membranes). Children don't metabolize chemicals the way adults do, so should use no more than a 10% concentration; those under six months old shouldn't use it at all. Effective repellents can be very hard to find in many places, so be sure to bring what you'll need; it is the best protection available against insects that transmit serious diseases, such as malaria, Lyme disease, and Rocky Mountain spotted fever.

If you're concerned about getting bitten/stung by any of the flying, crawling, & swimming creatures capable of doing so, you might consider carrying a small Aspivenin® pump. Used by the French Army, Médicins sans Frontières, UNICEF, Wildlife Conservation International, etc., this clever mechanical device (developed by André Emerit, a French inventor who died in 1997) visibly extracts venoms without damaging the surrounding skin. You can learn more about it, and order one on-line for about $20, from Aspivenin (France) or from Flairpath Marketing (UK). Sawyer Products sells what appears to be a cheaper (in the U.S.) copy of this device: the Sawyer Extractor.

sunscreen, lip balm (because malignant melanoma is so depressing) Remember, sunscreens work by penetrating the skin to create a barrier, while insect repellents work by evaporation, so apply the sunscreen first. Be aware that sunscreens more than 2-3 years old have lost their effectiveness. Decent lip balms are hard to find in most areas of the world, so bring enough to last you.

Lots of walking can lead to chafing, which is easily remedied if you have a bit of moleskin, and a potentially serious problem otherwise. In bandages, I like the cloth butterfly-shaped style (designed for fingers/knuckles): they can handle surprisingly drastic cuts, and are easily trimmed down to smaller sizes as needed.
menstrual, contraceptive, and/or prophylactic supplies Women traveling for extended periods of time might want to give thought to alternative forms of menstrual supplies. Tampons, for example, are difficult to find in many countries, and often prone to be "toxic shock" specials. Travel- (and environment-) friendly alternatives include menstrual cups (such as the DivaCup, Keeper, and Mooncup), which sell for around $35 and last for ten years, and reusable cotton pads (such as GladRags and Lunapads).

Condoms brought from home are less likely to break (and more likely to be available). Remember, the diseases most often caught during foreign travels are sexually transmitted ones.
Vitamins, necessary medications if you need to carry any prescription drug, be sure to bring along a copy of the prescription, both to replenish your supply in an emergency, and to verify your need for something that might otherwise be illegal in your destination country. The drug should be listed by brand and generic name, and include the dosage. If you have a serious condition, a letter from your doctor (translated into the local language) is a wise precaution. Carry more medicine than you expect to need, especially if traveling somewhere without Western medical standards;

Have your prescription filled in tablet form when possible, as capsules and liquids are less stable, particularly in hot, humid climates.

Avoid buying prescription medicines abroad, especially in Third World countries, where local pharmacies will compound drugs themselves, perhaps using something like sulfa (to which you may be allergic) as a filler. Counterfeit drugs are also a problem (as are expired drugs "dumped" from the U.S.), particularly in Mexico, Thailand, South America (except Argentina & Brazil), Africa, and Hong Kong.
Plastic water bottle, (folding) cup Drink plenty of clean water, like your mom told you (or should have). That's not always easy, depending on where you are (like a plane, or other than an urban portion of a developed country). So carry your own, in your daypack. A recent innovation is the Platypus® collapsible water bottle, which I have found to be surprisingly rugged. Made by Cascade Designs, and available from REI and Campmor, these solve a longstanding problem of efficient water storage. Magellan's sells a nice, leakproof, folding cup (which can also be used for cleansing purposes in squat toilets); if you're planning to use an immersion heater, though, you'll want something less likely to melt (and also, alas, heavier & bulkier).

Audiovisual (Use the red bookmark menu on the left for other sections.)
dark glasses, retainer, case Sunglasses will save your sight for your retirement years (get good ones, with 100% UVA and UVB protection). The eyewear retainer ("Chums" is a good brand) and case may save your glasses.
lens cleaning cloth/supplies, copy of prescription, (spare glasses?) If you wear prescription lenses, and don't carry spare spectacles (or even if you do), a copy of your current prescription could prove invaluable. Cleaning supplies are of particular importance to contact lens users.
earplugs (eye mask?) Earplugs can be extremely useful, not only at rock concerts (to save your hearing for your retirement years), but in situations where you're trying to get some sleep, and the party next door (or on the street) isn't. In noisy third world cities, these are pretty much a necessity. I like those that you compress by rolling between your fingers (they expand in your ear canal). If you're planning to sleep on a long flight, you may also find an eye mask helpful (Eagle Creek's "Comfort Eye Shade" is an effective design that eliminates pressure on your eyelids).

Pens, small notebook, and glue stick Most journeys will yield interesting items of information along the way (addresses of people you meet, the locations of hotels, restaurants, and other recommended attractions, a record of photographs taken, even a trip diary); make sure you bring something in which to write them down. It can be as simple as a small coil-bound notebook, or elaborate as a hardcover journal into which you glue the ticket stubs, business cards, and other memorabilia that will recall your adventures in years to come.
Personal address book (stamps?) Don't forget to send postcards to those stuck back home; remember that the stamps from your home country won't work in a foreign one. Some people like to carry pre-addressed envelopes, but I've always preferred buying postcards as I go. Another solution is to bring along pre-printed address labels for all your intended missives, and affix them to cards or envelopes as necessary (this has the added benefit of keeping track of what you have yet to send). Obviously, all of this depends on how much you plan to write. Even if you're not planning to write to anyone, it might be prudent to have telephone numbers (home numbers, if possible) for your doctor and travel agent; they just might help avert a disaster.
maps, guidebooks, phrase books, Post-it® notes, restaurant lists, membership cards, business/calling cards, telephone access numbers These can be important, but also a challenge to your "packing light" goal. Don't carry maps that can easily be acquired en route. Instead of the whole guidebook, perhaps you can cut out — or photocopy — only the section(s) pertinent to your needs, discarding no-longer-required portions as you travel. Or simply summarize the pertinent information in your notebook. Consider replacing a collection of foreign phrasebooks with a Kwikpoint card, containing over 600 universally recognized symbols designed to get your message across. And a pad of the smallest Post-it notes will serve as excellent page markers for your guidebooks.

Another good reason to use local maps in place of those you have brought from home is that the former will have place names in the local languages, rather than "translated" into English. This is particularly important if you are touring by car, as you are unlikely to see Prague, Florence, or Munich on road signs (think Praha, Firenze, & München).

Long distance phone calls can be expensive from foreign countries (for that matter, any phone calls made from a hotel room are likely to be very costly); come prepared with a list of access numbers to reduced cost services. AT&T provides a wallet-sized card listing their own such numbers (in the U.S., call 1-800-331-1140 for a copy, or print one from their Web site). Even better is the use of international phone cards (with Personal Identification Numbers), widely available at newsstands and the like in industrialized countries.

I use resources such as VegDining.com to compile a restaurant list prior to any sojourn into unknown territory, and carry my Ocean Cruising Club card, which gets me into foreign yacht clubs. Business/calling cards are an ideal way to leave your address(es) with newfound friends, and are occasionally useful for impressing local officials. Finally, consider carrying a photo or two of your family & home, and possibly a few postcards of your city; these help establish your identity as a "real person" to those you meet along the way.
reading material If you're a reader, travel prepared; reasonably priced books in your preferred language are not always easy to come by in foreign countries. Choose paperbacks with dense print; discard (or trade!) them as you go.
large envelopes One way to keep your bag light as you travel (especially on longer trips) is to mail accumulated stuff home; having a few large manila envelopes (or better, those made from Tyvek®) both aids and encourages this.

Documents (Use the red bookmark menu on the left for other sections.)
Passport, visas, extra passport photos, vaccination certificates Take what you need (find out what you need before you go). The extra photos are for visas you might decide to obtain along the way.
Copies of important documents If you should manage to lose your passport, a photocopy of the main page will make replacing it a lot easier. Similarly for prescriptions, charge/ATM card account numbers, traveller's cheque serial numbers, etc. Of course you'll carry the copies separate from the originals, won't you? It's also a good idea to record the special telephone numbers necessary to deal with the loss of any charge/ATM cards you carry; leaving an additional copy of all this information with a trusted friend back home can save you a lot of long distance calls in case of disaster (many 800-type numbers don't work very far from home in any case).

Companies like Xdrive will provide you with inexpensive Web-accessible storage, so it's possible to store important information that can be accessed from any Web browser. Given the dramatic rise in the availability of Internet Cafés, you may find this a useful repository for much of your backup information.
(International?) Driver’s license, health insurance information In general, you'll want to leave at home most of those cards you carry around in your wallet; leave the wallet at home too. But there are exceptions. Should you need to drive, your regular driver's license (in conjunction with your passport) will almost certainly be sufficient. In theory, some countries (particularly those with non-Roman alphabets) nominally require "International Driving Permits" (IDPs), which are simply translations of your regular license into ten languages; in practice, most officials have never heard of them. If you're unsure, though, check with a local authority (such as the rental agency you intend to patronize). The "International Driving Licenses" one hears about (especially via the Internet) are essentially a big scam; before sending off your money for one, read this U.S. Federal Trade Commission report, which also has information about to obtain a legitimate IDP.
Travel tickets (i.e., airline tickets, rail & bus passes, hotel reservations, etc.)
Charge & ATM cards, cash, traveller's cheque It used to be that traveller's cheque were the preferred method for carrying money; this is no longer the case. For purchases, use charge cards whenever possible (in much of Europe & Asia, this includes even taxis & fast food outlets); you'll get a much better currency conversion rate (generally 1% over the commercial bank rate) than by any other (legal) means. You'll also postpone the final reckoning. Make sure that your cards have sufficiently high credit limits to cover what you'll need. It's also advisable to call your credit card company prior to travelling far from home, to let them know your plans; this will save you the inconvenience (and possible embarrassment) of having your card refused because the computer doesn't know you're in Bangkok.

Be aware that some charge card issuers (as distinct from the networks, such as Visa and MasterCard, which make their money from the 1% mentioned above) levy extra fees (two to four percent!) for foreign currency conversions. If your card is one of these (find out before you leave), change cards (and let them know why you did). Really. Alas, I don't have any easier way than you (by phoning around) to determine which cards currently levy extra charges. I don't try to keep track, as it changes without notice, and I'd hate to be posting obsolete information.

To obtain local cash, use a bank ATM card (make sure yours is part of of one of the global networks, like Cirrus or Plus); both MasterCard and Visa have Web-based ATM-finding services. It's better not to use credit cards for this, as you'll incur a cash transaction fee and begin accumulating interest immediately. Be aware that 4-digit Personal Identification Numbers (PINs) are standard in most countries; should yours be longer, arrange to get a new one. If your bank levies a charge for foreign ATM transactions, don't withdraw $20 at a time. Note that an American Express card also acts as a cheque guarantee card, allowing you to cash a personal cheque at any American Express office (making this the best non-ATM way to transfer money from home).

Yet another solution allows you to bypass most of the charges that are sometimes attached to the use of ATMs. This is the so-called stored value card, available from Visa and also AAA. It's sort of like a telephone card: you load it with money and can then collect that money from ATMs along your way, paying only the local dispenser fee.

If you do use traveller's cheques, bring more than you think you'll need, in both large and small denominations (some currency exchange places charge by the monetary amount of the transaction, some by the number of cheques converted). Stick with "major labels" (e.g., American Express or Thomas Cook). As a rule, banks (and American Express offices) will give you a better deal than the many late-night bureaux de change you will encounter (the "no commission" signs merely indicate that their cut is built into the exchange rate; conversely, good exchange rates imply high commissions). Keep some cash handy for essentials, but any significant amount safely stored (see "security pouch" below).

Pay attention to your money. Understand all charges included in any transaction (ask to have bills itemized). Particularly in highly touristed areas, assume that you'll be shortchanged; always ask how much, do your own arithmetic, and don't let yourself be hurried. Be friendly, but vigilant.

If you're travelling in third world countries (where traveller's cheques and charge cards are largely useless), carry lots of American one-dollar bills; hegemonist issues aside, you will find them to be the closest approximation to a universal currency. A couple of hundred bills will not fit comfortably in your security pouch, so you will have to secrete most of them in various places about your pack (don't fret excessively about this; the loss of your pack will prove more of a disaster than the loss of a couple of hundred dollars).
2 personal cheques You may not use these, but they take almost no space, help establish credibility, and just might prove invaluable. Bring more on longer trips, should you plan to use them to transfer money from home (as mentioned under "charge & ATM cards", above).
security pouch I can't overemphasize the necessity of this; it may be the single most important item on this list. Street crime is rampant in most parts of the world; to compound the problem, you will usually find it difficult to avoid looking like a tourist (read "target"). So never, ever, put any of the items (except document copies) from this section of the list anywhere but in a security pouch, worn under your clothes (and not accessed in any public place).

These pouches come in a variety of styles; the safest are those worn around the waist. The type that loops around your belt (assuming you're wearing one) and hangs down inside your pant leg, is almost as safe, and somewhat more convenient and comfortable. Look for light weight, and sufficient dimensions to hold what's necessary. Mine (the World Class Passport Carrier, from Easy Going) even has thin aircraft cable sewn into its belt loop and waist strap to foil the thief who would cut it off (though you should wear it under your clothes anyway). Get one of these, and get used to wearing it; it's the cheapest insurance you'll find.

If you need a little more incentive to use a security pouch, you might read Terry Jones' illuminating Street Scams of Barcelona. Not to be viewed as an indictment of Barcelona, it's typical of the sort of activity that abounds at many heavily touristed areas around the world (some are much worse!).

Finally, in these days of increased airport security, with official/officious folks checking our identification/tickets/receipts/whatever at every turn, some travellers have found it helpful to wear a handy neck pouch to hold all of these, making them instantly available when necessary (and recalling days in kindergarten when you wore a name tag around your neck). These are not the equivalent of a proper security pouch, and should not be used as such, but they can reduce one's stress level when en route via air. Easy TravelAir makes a very nice version of this device.

Items in this section definitely fall into the "optional" category, the need for same being very much a function of one's personal interests and the nature of the trip. This part of the list could, of course, be arbitrarily long; I have restricted it to the few items that are of extremely broad interest.
camera (lenses? flash? tripod? film? extra batteries? charger?) I'd recommend a state-of-the-art "point and shoot" camera, unless you really want to lug that SLR around. Philip Greenspun offers an extended and helpful discussion of this topic.

If you're using a film camera, be aware that film can be quite expensive outside North America; if you travel from there, it's often best to carry what you think you'll need. This decision, however, must be weighed against the damage that will be done to that film by passing it through airport X-ray machines. I know, they tell you it won't damage your film, but it will. That damage is unlikely to be visible if you're using slower speed film (anything under ASA 400), and the film only suffers a single exposure. With higher speed films, though, and/or multiple X-ray exposures (radiation damage is cumulative), you're taking a chance. Ask for hand inspection of film, though this request wil not necessarily be granted (especially outside North America, where the X-ray dosage tends to be higher as well). Lead-lined bags are available to shield film, but they're heavy and space-consuming; there's no easy solution.

The digital camera user is spared the difficulty of coping with film, but must instead deal with batteries. Choosing a camera that uses a standard battery (especially the ubiquitous AA size) will help considerably, as will the use of high-efficiency batteries and chargers (unlike those that are typically bundled with the original camera). Thomas Distributing is a good source of such technology. Consider also the use of higher-capacity memory cards; one can capture a couple of hundred photos per 256MB card, greatly reducing (even eliminating) the need for downloading during your trip.
cellular telephone (charger?) Travel is often best accomplished in the absence of "instant telephony", but for many, cell phones are a business trip necessity. Before you depart, ensure that the phone's technology, your calling plan, and the battery charger are all compatible with the area(s) you intend to visit.
(solar) calculator Depending on how arithmetic-challenged you are, one of these might help you with currency conversion. Some are available with quite impressive foreign language dictionaries built in. Get a solar-powered design to reduce the need for batteries.
gifts A final suggestion is to think about taking along some small items that you can present to people you encounter along the way, especially the children (there is no faster way to endear yourself to the families you meet). The colourful (economical, flat) press-on stickers you can generally buy near gift wrapping supplies are appropriate for many ages. Another good idea is to carry a supply of the balloons used to make animals, hats, and the like. You've almost certainly seen this done, and perhaps never realized how easy a skill it is to learn. T. Myers Magic will provide the instruction (including how to blow up the balloons without popping your eyeballs) and the supplies; see the resource section.

For adults in less industrialized areas, inexpensive (but not cheap, throwaway) ballpoint pens are a good choice; they also make good bargaining chips at markets. High quality fish hooks will be very scarce in poor fishing communities, making them excellent gifts and trade goods (especially for fish!); make sure you get the flat variety. Bandannas, not widely available outside North America, are another good choice. Other adults may appreciate a souvenir (perhaps a small flag pin) of your home country; as a Canadian, I often carry some of our $1 gold-coloured "loonie" coins (so named for the loon pictured on its reverse side).
A reminder... contact information, when available, is provided for the various products & manufacturers mentioned here. You'll find such information on the Resources page.
Perfection is attained, not when there is nothing more to be added, but when there is nothing more to be taken away.

In the aftermath of the World Trade Center and Pentagon attacks, the list of items that may not be brought into aircraft cabins has grown considerably longer, and the actions of security personnel more zealous. Although the Transportation Security Administration (part of the U.S. Department of Transport) publishes an official list of both prohibited and permitted items, their existence doesn't necessary predict what will happen at the airport, where the personal views of random security personnel may have a greater impact on your travel experience.
Nonetheless, you should familiarize yourself with the official position, to ensure that you are not inadvertently attempting to board with something untoward. It's also advisable to revisit the list occasionally, as it changes from time to time (nail files, for example, were not always permitted). Finally, appreciate that the above list is only definitive for the United States; other countries may have their own rules, which are not necessarily the same. Australia forbids blunt-tipped scissors, corkscrews, and knitting needles, for example, all of which are allowed in the U.S.

Things that must be plugged in are conspicuously absent from this packing list; they add bulk and weight rarely compensated for by usefulness. Try hard to eliminate them. If, however, you just can't live without your electric shaver (a small bottle of shaving oil and a razor are much more appropriate), or your hair dryer (think about towel/air drying, or even a more travel-friendly hair style), don't forget that electrical power varies throughout the world. Most countries use 220 volts at 50 Hertz, though several (including the U.S.A.) use 110 volts at 60 Hertz; moreover, there are more than a dozen styles of wall socket, each requiring a different type of plug. You also need to worry about power (wattage) ratings.

Plug adapters and power converters are available for all of this, of course (Magellan's is a particularly well-equipped and helpful source; also check the "Useful Tools" section of the Travel inks page for an excellent information site), and some products are available in compact travel models with multi-voltage settings (you'll still need plug adapters). Business travelers (who are likely to be carrying laptops and their attendant power converters) should know that electrical items such as battery and cell-phone chargers can be obtained in USB-powered versions, eliminating the need to carry additional (heavy, bulky) converters. But you can best eliminate the hassles by eliminating the gadgets. Even if a hair dryer does make a good sock dryer.

All of this notwithstanding, however, a simple immersion heater is one appliance that many travelers (especially those on a limited budget, trying to avoid restaurant meals) consider invaluable. Place it in a cup of water, plug it in, and in almost no time you have boiling water. This not only kills all disease organisms, it also lets you make a hot cup of tea, coffee, cocoa, soup, etc. You can even cook an egg (bring the water to a boil, remove the heater, drop in the egg, and wait a few minutes). Small, lightweight immersion heaters (such as the Franzus IH100 model pictured here) are inexpensive and available in dual voltages, though you may still need a plug adapter (alternatively, you can buy a new one locally in most developed countries). Better-quality versions of these have thermostatic shut-offs; with cheaper ones, be sure that the coil is immersed in liquid whenever the unit is plugged in!

I know... children aren't packing-list items, but a lot of people ask about traveling light with children in tow. With toddlers and smaller (who will need diapers, favorite toys, etc.) it's a daunting challenge. I suggest renting a car. If you have a choice of destinations, choose those where you are likely to want a vehicle anyway (Ireland, say, or the Provence region of France, where you will want to visit areas not well served by public transportation). Note that foreign rental cars should, if at all possible, be booked in advance (from home); doing so after you arrive can be much more expensive. Your chosen guide book will often suggest lower-cost local alternatives to the major auto rental companies. Don't assume that theft and collision coverage on rentals works the same way as at home; your own policy is unlikely to be valid in foreign countries, and the automatic coverage provided by some charge cards is also very country-specific, and should be verified in advance.

If you are traveling with young children (even if only to the local market), you should know about child carriers, designed to comfortably transport young ones on your back. If you lead any kind of active life whatsoever, you'll never regret acquiring one of these. Several companies make them, but Tough Traveler® (pictured here) is the brand to beat (contact information is, as always, on the Resources page); they've been building them for over 20 years.
Once the children are old enough to keep up with your walking plans, they should of course be introduced to the joys (and responsibilities) of independent travel. This means they should have bags of their own, in which to carry whatever they choose to bring along (in practice, this will primarily be clothing, as the accompanying adults will already be bringing all the communal needs). A modest daypack should suffice for this; if possible, you should shop for same with its intended user... not only will s/he be involved in the purchase, but it can be examined for proper fit.

Finally, you'll find lots of good information and products related to traveling with babies and toddlers at BabyGoes2.
A Few Closing Words...
People overpack because of timidity and fear of the unknown, both largely results of inexperience. I've written these pages to provide — in some measure — the benefits of a not inconsiderable amount of urban/rural and business/tourist travel experience. How will you know if your ultimate choices are good ones (preferably prior to that once-in-a-lifetime trip)? Fortunately, there's a simple test...
Pack your chosen bag with the items you plan to take. Put on the shoes you plan to wear. Then take a one-hour walk around town, preferably including an ample assortment of stairs, poor roads, and the like. If you're still happy at the end of your excursion, make your travel reservations.