Canadian Airlines

Air Canada
Tango.......Mtl ---> Florida, Toronto and west. :::::::: Tor ---> also Halifax, Fred.
AirTransat.
WestJet........ Mtl ---> West, Hamilton, Halifax.
CanJet......... Mtl ---> Ottawa, Toronto, and East.

European Airlines

Here's a cool site pointed out to me by John Hostetler: AirNinja.com . You can check out which budget airlines fly to/from your area.

EasyJet. Cheap fares, but not always.
Ryan air. Cheapest fares usually. Even within just a few days notice.
Virgin-Express
Air-One (mainly within Italy, goes to ATH from Rome-Milan)
Air Dolomiti
Aegean airlines.
Olympic.
Other useful companies not linked to yet: WizzAir, SN Brussels Airlines

SearchEngines

Travelocity. Useful.
Destina. Now just an AirCanada server.
Travel cuts. Worth a look, especially for students.
itravel2000. Mainly for flights out of Ontario.