Largely aimed at the business traveller, the Bradmans Web site lists basic essential information (including airport-to-city instructions) for over 100 cities worldwide.
The U.S. Government's Centers for Disease Control and Prevention maintains a superb Web site, filled with health information related to international travel. This is the place to discover what you need before you go.
Various foreign affairs offices provide useful (and important) information about travel in other countries, including but not limited to foreign entry requirements (passports, visas, innoculations, etc.) and travel warnings (political unrest, lawlessness, violence, natural disasters, epidemics, aircraft safety, etc.). For example, Consular Affairs supplies this information for Canadians, the Foreign and Commonwealth Office offers it for U.K. citizens, and the State Department does so for Americans. It can be useful to compare their respective opinions.
Wonder what your money is worth in some other currency? A great place to find out is OANDA's wonderful Currency Converter. This site will even create a customized, wallet-sized "cheat sheet" for you to print and take along!
If Europe is in your travel plans, you'll find useful reading (and copious advertising) at European Visits, "The Online Magazine of European Travel".
Steve Kropla's well-maintained Help for World Travelers has what is likely the most current country-by-country listings of: electrical information (plugs and power), international telephone codes, & phone/modem plug standards. Other good stuff too.
If you're roaming the world with a laptop, searching for a network connection, JiWire's Wi-Fi Hotspot Finder might be just the thing you need: tens of thousands of wireless hotspots, free and otherwise, in about a hundred countries.
Travelling the world without your computer and missing that regular Internet fix? Wishing you could send some e-mail back home, or pick up messages from friends? My brief introduction to Internet Cafés suggests what in most cases is the ideal solution.
Produced by Time Out, "one of the planet's leading publishers of entertainment listings and city guides", this au courant site suggests where to go and what to do in over twenty of the world's major cities.
The nearest government tourist office, visitor's bureau, or the like is generally an area's best source of local information; for tourists, it should be the first stop when you arrive at a new place. The Tourism Offices Worldwide Directory is a bit plain, but has plenty of listings of such non-commercial services.