-Pen and Paper. I keep a pocket notebook and a small pen handy everywhere. It saves a lot of time and trouble when trying to remember names of places, things, people, or just take notes. Also good for swapping contact information with any new friends along the way.
-Napkins. I always keep a stash of tissues and paper napkins in my back pocket. They come in useful at odd times, whether you just need to blow your nose, wipe your fingers and face off after eating, or try to get something off your clothes. You'll be surprised how often people ask for these. Also, in countries where the concept of toilet paper hasn't really caught on, it can help you in a pinch.
-Two Anti-Diarrhea Pills. I usually use what we call Imodium in the US-- Loperamide Hydrochloride is the real name. Hopefully you won't need it, but if you do, the sooner you have it, the better. I don't think I need to go into details why, aside from saying multiple hour bus rides can be really unpleasant in the wrong circumstances.
-Condoms. If you're even vaguely considering being sexually active on your trip, this is an absolute must. Very nasty STIs are much more common than you'd think in just about any country in the world, developing or developed. Other travelers from your home country aren't any less risky sexual partners than locals-- you don't know who travelers have had sex with where. Plus, if you think an unexpected pregnancy would be awkward when the parents live in the same country, try parents living of opposite sides of the planet. Don't risk it. Bring condoms from a brand you trust from home, as reliable ones aren't always readily available abroad (only resupply with brands you know and trust), and make a habit of keeping one on your person. Even if it seems unlikely that you'll use use it, it's much better to have one when you don't need it than to need one when you don't have it.
-Rubber Bracelet/ Hair Ties. I didn't pick up on this one until several months into the trip. You know those yellow "Livestrong" rubber bracelets that Lance Armstrong made so popular a few years ago? They still make knockoffs everywhere, and they turn out to be pretty handy on the road. Travel agencies hand them out in Australia as free advertising and charities around the world give them away in exchange for a small donation. You can even get orange ones that say "long live the king" for cheap in Thailand. People with long hair can use elastic hair ties for the same trick. Mostly I use them to keep food containers closed or to hold something together if it unexpectedly breaks, but there are weird situations that come up where having a rubber band on hand is really useful (I think the most unexpected was during a pub trivia night in Australia that included a random marathon competition with glowsticks-- I don't even remember why having several stuck to my wrists got our team so many points, but it did). If you have long hair, hair ties can be used for the same purpose.
-Pocketknife. This one's optional, I haven't found it as useful as most other backpackers make them out to be, and it's a liability whenever I go through any kind of security checkpoint. Still, it's good for quick repairs, odd jobs, and peeling fruit. Also makes me feel a bit safer when doing something like hitchhiking (though, unless you have training, I don't think it would help much in a fight, especially if your opponent is also armed). If you do get one, make it a cheap one that you won't mind surrendering to airport security if you forget you're carrying it.
-Lighter. Especially useful for solo non-smokers. Yes, you read that right, non-smokers. Smokers the world over speak your language and can't keep track of their lighters for more than five minutes at a time. In some backpacker hangouts, asking for a light is practically a running joke. I don't smoke, but I carry a lighter so that I can meet other people. In countries where smoking is common (or in places with a lot of European travelers) this comes in handy. In fact I would go so far as to say that if you are a solo traveling smoker, consider going without a lighter. It will force you to ask for one and meet other people. Also might help you if you're trying to quit. The lighter can also good for repairing thin broken plastic (melting parts together) and lighting mosquito coils, incense and candles. Same as the knife, buy a cheap one you won't mind surrendering to security somewhere (though surprisingly this doesn't come up very often).
Finally, there is one little piece of equipment that I don't always have on my person, but that I count as one of the most versatile and useful things a traveler can have anywhere (especially considering how little it costs). In the Hitchhiker's Guide to the Galaxy, it was a towel-- you could do things like "use it to sail a miniraft down the slow heavy River Motht" or "wrap it round your head to avoid the gaze of the Ravenous Bugblatter Beast of Traal (such a mind-boggingly stupid animal, it assumes that if you can't see it, it can't see you)." Since I have yet to see a Bugblatter Beast or a river I could float down on with a towel for a sail, I've got something more practical in mind.
But I'm not telling you what it is yet. I'm not doing this to be a tease, I'm doing this because I'm curious as to what other people will guess. I want you to submit your ideas. There's a good chance that even if nobody gets what I'm thinking of, they'll come up with something else I wouldn't have thought to include, or maybe something even better. I'll post my answer in my next tip post. Guesses, anyone? Comment on the post or send them in by email. If you're reading this by email subscription, the comment link will be at the bottom of the post on the main site. If you're reading this on Facebook, try the main blog. I won't be able to see Facebook comments as China has blocked access to Facebook for a while now. If you can't comment on the post for some reason (people have been emailing me saying they couldn't on my last entry) send me an email and I'll get it up on the next tip post with my answer.