Nobody's mentioned Godspeed You! Black Emperor and their spinoff bands yet (A silver Mount Zion, Fly Pan Am, Hrsta, etc etc) as well as the rest of the Montreal scene. The best post-rock available. I think Do Make Say Think are canadian as well. A Silver Mt. Zion are on tour now (UK and later US), should definately see them if they're in your area.
Also worthy of a mention are
The Hidden Cameras
The Arcade Fire
Neil Young (!!!)
And if you like techno (or even if you don't), check out
Jesse Somfay, a young kid that made one of the very best songs of 2005 - This Fragile Addiction.
Canada is a great land musicwise.
EDIT, well i guess someone did mention GYBE before. They're worthy of being mentioned again.