Kayak
A slew of new features makes this a compelling meta-search site useful for planning your next trip. You can search airfares to different cities simultaneously (click "multi-city search"); Fare Watch lets you track any number of trip itineraries and receive email alerts when a fare meets or beats your specified price. Kayak Buzz lists the best fares to 25 popular destinations from a specific airport, and now you can also view the "best fare history" — the 100 best prices found by other users searching the same route during the last 36 hours. The new Flexible Search lets you compare fares on alternative travel dates. Registered users can save preferences under My Profile, so they don't have to enter certain data on every visit. Of course, it pays to check an airline's FlightStats before booking — the site issues monthly reports showing how well carriers managed to stick to schedule. And once you've decided on a flight, head to Seat Guru for a detailed look at your seat-assignment options.