The World Health Organization has information about current outbreaks of infectious diseases worldwide, all innoculations required to enter each country, and travel health info on subjects such as Malaria. Click on the country list to find info about the places you are going to.
US Centers for Disease Control: Publishes outbreak warnings, suggested innoculations, precautions & other background info for travelers. They also run an international travelers hotline at 1-877-FYI-TRIP.
Travel advisories & warnings are listed here as well as entry requirements for Americans to all countries.
OVERSEAS CITIZENS SERVICES: 1-888-407-4747, from overseas: 202-501-4444 Death of an American citizen abroad, arrest/detention of an American citizen abroad, robbery of an American citizen abroad, American citizens missing abroad, crisis abroad involving American citizens afterhours number for an emergency involving an American citizen abroad.
The British Foreign & Commonwealth Office also issues travel advisories and has all entry requirements for UK citizens, they also have a number of PDF booklets on safety-type topics.
The Australian Government's site for documentation, travel warnings, and advice.
The New Zealand Government's site for documentation, travel warnings, and advice.
World Meteorological Organization: For up to date forecasts or to check the weather conditions at any time of the year in a certain county.
Worldwide listing of public holidays. (So you won't do what I did and plan to take the train on a holiday, then wonder why the train never comes)