Traveling to Canada this summer...what is a good website to book hotels in Montreal, Vancouver, and Toronto?

Any of the big online hotel reservation sites should do the trick:
http://www.expedia.com/
http://www.hotels.com/
http://www.orbitz.com/

Might also want to check the following, as you can easily see all the hotels in an area, then sort available results by price, etc.:
http://www.resideo.com/ca/quebec/montreal/
http://www.resideo.com/ca/britishcolumbia/vancouver/
http://www.resideo.com/ca/ontario/toronto/