I regularly use Kayak as well as Mobissimo and Side Step. They are more useful than Expedia, Orbitz, Travelocity for finding the lowest fares since they check not only the regular airlines, but also check the web sites of low cost carriers like Southwest and some consolidators. Problem is they only give you fares, and you then have to check for availability at the time you want. Also, they may list several hundred, up to a thousand fares, and it may take some time to sort everything out. Still, tremendous resources.

http://flights.sidestep.com/air http://mobissimo.com/travel/search_airfare.php

Bruce