I also check Orbitz, and the airline websites directly. I really haven't had much luck with middlemen-type companies, though and mostly buy direct. We have, upon a couple occasions, had luck with
http://www.justfares.com, an online consolidator.
Unfortunately, their site is a pain to navigate, and you have to make a "reservation" (no credit card, you just have to give them names, addresses and they bug you if you don't end up taking the booking) in order to see any flight information. You can talk to a live agent, but we've had varying degrees of success with that. If you log on and end up with Roger, run awaaaaaaaay! Perhaps calling them directly would be better. Prices are usually pretty good, though. A couple years ago, we got a roundtrip airfare from Dayton, OH to London for a little over $300 per person.
I've always heard good feedback about
http://www.go-today.com, but they sell packages, not individual flights, IIRC.
Ariane