I'm assuming you're looking for a web-based radar map as opposed to an installed application.
The only radar map I've seen that aggregates the US NEXRAD and Canadian Radar systems is:
However, that being said, the US NEXRAD radar covers almost all of Southern Ontario.
Two more useful NEXRAD-based sites:
(reflectivity only on composite view)
(reflectivity and doppler)
Hopefully this helps.