Flight search, owned by Google, searches a month's worth of dates, not quite sure I see the point, but trusted folks rave about it.
Some crazy cheap flights if you get the rights date / destination combo.
Cheap flight search, in French only, but seems to cover some airlines that SkyScanner didn't. Nathaniel found ultra cheap flight from Marrakesh to Marseille for example.
Search flights and see them laid out in a grid that shows take off time, layover time, stops, and sorts according to "agony" which tries to factor price, layovers, etc.
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.