Kayak is like Google for travel sites. Its search engine scours the Web to help you find the cheapest deals on travel services around the world. It searches more than 140 travel sites and lets you sort and customize the data. You don't actually purchase the tickets or hotel bookings through Kayak, it simply directs you to the purchase page.
Some of its most useful features are the Buzz and Fare Alert planning pages. Buzz shows you the cheapest deals as discovered by other Kayak users in their searches. These prices are aggregated from historical data and listed when you do a search from your city. The Fare Alert service will send you an e-mail when a trip from one city to another becomes available at a low price, letting you pick when you're going to travel by the cheapest price.
Winner: Kayak (Kayak.com)