Links to other (hopefully) useful sites:

U.S. State Department:
These are the folks that keep track of U.S. citizens abroad, communicate with embassies, and airlift your ass out if you get in trouble (at your expense, of course). It's good to know what they're saying about your destination and it's probably wise to follow their advice. They've got a wealth of info on this site, including passport info, visa info, tips for travelers, travel advisories, etc. Among other things, check out their Foreign Entry Requirements and Travel Warnings/Advisories and Consular Information Sheets.
World Atlas
This is an excellent collection of maps all over the world. A good link to give to everybody you're leaving behind, so they can see where you are.
Premier Travel
A travel site with lots of links to all kinds of travel resources. I have no experience with their commercial services, but they have a lot of useful links.
International Bike Fund
This guy wrote the book (literally) on biking through Africa. Lots of good info both for cyclists and non-cyclists.
Travel-By-Cycle
An excellent site dedicated to bicycle touring. Lots of links, lots of resources. I used this site a lot.
OANDA Currency Exchange Converter
In some countries, the value of your currency (which, if you can afford to be traveling, is probably "hard" currency such as dollars, pounds, francs or deutschmarks) can fluctuate wildly with their "soft" currencies. Inflation in some countries is more than 100% per annum, so the rates published in 2-year-old guidebooks are worthless. There are dozens of sites for currency conversion, so I basically picked this one at random. The OANDA converter is pretty cool, though, as it lets you select your means of conversion (credit card, cash, etc.) and estimates your average commission rate, figuring that into your conversion (if you wish).
World Flag Database
What exactly DOES the flag of Estonia look like? Find out here.
Center for Disease Control
These folks keep track of outbreaks of nasty things that you don't want to catch.
World Health Organization
Another good resource for finding out who's dying of what these days, and what's available to keep you from getting it.
Newsgroups
USENET: A "bulletin board" where you can post messages and read what other people have posted. It's mostly garbage these days (ads for mail-order brides and sex videos), but if you sort through all the crap, you can find some very useful info. The real wonder of the USENET is that you can post a question, and anybody in the world with access to that newsgroup can read your post and respond to it.
Search Engines
Find whatever you want (with a limited degree of success; check out each site's hints for writing a successful query).
Web Design Resources
Want to make your own site? These folks will help you learn. I used these in designing this site.

If you have other links which you think ought to appear here, or if you run a (related!) site which you think we might want to link to, contact me.