I use orbitz to search for flights across many airlines. I usually end up purchasing fron continental.com because I'm a one pass member and can cancel without penalty within 24 hours.
The orbitz guarantee is kind of shady, if you ask me. Someone else has to book your EXACT flight (both legs of a round trip on the same dates on the same flight). It's pretty unlikely that someone else will be booking the EXACT SAME trip as you.