I personally like
Hotwire I have found that they have th lowest fare over Orbitz, Travelocity, Cheapfares, and Expedia.
I like hotwire 'cause I can check the option for red-eye flights. They give you a choice of airports and fares, so you can pick the lowest avalible. My only complaint is I've been put on American Airlines a coule of times. I hate AA with a mad passion but a cheap fare is a cheap fare. AA is notorius for leaving /arriving late and just bad attitude. Both of which I experienced on every flight I have taken with them. Other than that, the airlines they put you on are all top airlines. Nothing like National, or Southwest...
The only catch with hotwire is you won't know who your airline carrier is or what time you're leaving until after you make your purchase. So this site is good for people who are more flexible time wise. If you're in a time crunch and need to know the details, Orbitz is good.
BTW,
Hotwire also does car rentals and hotels, niether of which I have used.