Airport Transit Visa
Information on airport transit visa policies vary from country to country and may change over time. There is not one specific website which contains information about all countries' airport transit visa policies.
The best thing to do is to go to the embassy/consulate website of the country you plan to travel through to find its particular policy -- and then contact that embassy/consulate, since the wording on some of the policies can be confusing.
Embassy.org has a list of foreign embassies in the U.S.