I've recently become a fan of Mobissimo.com, which is like Sidestep -- only it's web based instead of an application and it searches various booking engines like Travelocity in addition to airline websites. It also searches international sites like Opodo and Zuji and consolidators like airfare.com and onetravel.com. ITA Software is indispensable for figuring out itineraries but unfortunately they don't book for you. Then there are useful tools like Expedia.com/pub/agent.dll?qscr=fpfl&&zz=1058751296746&&&zz=1084052231172&"]Expedia's Airline Fare List, United's 5% off link, Expedia.com/pub/agent.dll?qscr=cmpf&airp=&dair=&&zz=1057387736447&&&zz=1063908544979&"]Expedia's Fare Compare, Expedia.com/pub/agent.dll?qscr=cmpf&airp=&dair=&&zz=1057387736447&&&zz=1063908544979&"]Expedia's Lowest Fare Search, [url="http://dps1.Travelocity.com/lognlogin.ctl?tr_module=FARE"]Travelocity's Fare List, TravelstoreMaker, and FareChase. And Smile67's Availability Tool is indispensable. Others to add to the list?