I'm not sure what's available in Canada, but I've had good experiences with
Pandora,
Slacker,
Last.fm and
Lala in the States.
I've also had good success with the aforementioned
Grooveshark.
There's also a listing of
Canadian radio stations that stream their broadcasts online, including
CBC Radio and it's affiliates.
Thanks for the other ideas, guys... I'll keep
Shoutcast,
Live365 and
Songza in mind in the future.