Orbitz is a travel search engine like Kayak.com. I think they handle booking, which makes them much more reliable than Kayak who simply advertise fares.
TravelZoo.com receive paid submissions from travel companies which they claim to manually verify before listing. You can then search all of their listed deals via the site. Search was poor when I tried Cape Town.
Kayak allows you to search multiple flight / holiday web sites and filter the results using some very cool ajax.