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.
Mobissimo is a travel search enginge. You can search for flights, hotels, etc, etc.
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.
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! :)