Currency Cheat Sheet
Oanda.com has an awesome cheat sheet you can print and take along. Print conversions both ways and stick them together as a wallet card.
Country Info and Travel Warnings
The CIA Factbook: Regardless of how you feel about the CIA, they know a lot of stuff and they share some of it with readers.
The U.S. State Department also has a lot of interesting information. Check out their consular information sheets and travel warnings.
Vaccinations
The U.S. Centers for Disease Control has a thorough list of vaccinations recommended by location, as well as a list of authorized travel medicine providers throughout United States.
Weather
It’s always good to check just before you close up that suitcase. Mother Nature has a mind of her own and no one cares more than The Weather Channel.
World Clock
This site even has a converter to answer that always-challenging question: If I wait until midnight to call, what time will it be at home?