I've used Sky Auction.com two times with great results. First trip was from SFO to LHR with three nights at a decent hotel for two for around $300. The second trip was SFO to FRA on Lufthansa for two for a little over $500.

If you plan on using it make sure you know what a "good deal" is. Some auctions go for far more than the current fare on Travelocity or an airline web site. Also, do not bid early - that just drives the price up. Monitor the asuction you want to bid on and make your bid at the last minute and I mean last minute (if the auction ends at 3pm don't bid until 2:59pm).

Good luck.