Best website for finding cheap airfares

I’m always looking for better sites to find cheap airfares.  I get really frustrated that the airlines can charge not just different fares for different days but different fares for the same seats on the same airplane!  I found this site, ITA Software, yesterday and it’s great!  It even has an option "search within this 30 day timeframe for the cheapest roundtrip ticket assuming I want to stay 6-8 days."  Terrific.  You do need to register but it’s free.