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.
RedTag are a cheap flight / holiday search engine. Recommended by Canadians who found uber cheap tickets from Montreal to Belize City.
Budget Long Haul is a web site dedicated to flying budget around the world. It's run by the same guy as WorkingNomad.com (I think!)
Mobissimo is a travel search enginge. You can search for flights, hotels, etc, etc.
WhichBudget.com lists all the budget airlines that fly from country to country. You can search by departure and arrival, and they provide links to the various web sites. Damned handy! :)
JetBlue are a budget airline that operate across most of the US.