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.
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.
Some crazy cheap flights if you get the rights date / destination combo.
Live flight arrivals from flightstats.com for Cape Town (CPT) airport.
JetBlue are a budget airline that operate across most of the US.
Kayak allows you to search multiple flight / holiday web sites and filter the results using some very cool ajax.
Mobissimo is a travel search enginge. You can search for flights, hotels, etc, 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.
RedTag are a cheap flight / holiday search engine. Recommended by Canadians who found uber cheap tickets from Montreal to Belize City.
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! :)