I'm looking for a site that lists all of the big concerts that take place during August and September in Toronto, New York and Montreal.
Is there anything like that?
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While there are many dedicated and local sites that you can find for concert listings, I find that Last.fm's Events section usually has everything. Especially for large cities like New York, Toronto, and Montreal it will definitely cover everything, no matter how esoteric your music tastes are.
And if you 'scrobble' your listening to Last.fm, it can even help you narrow down the listings to artists you listen to more - because in any given month if you're looking at the complete list you'll be looking at dozen of entries for each week.
An alternative to last.fm, Songkick offers a similar feature where you can search cities for events, and add your own musical tastes to narrow the search.
For NYC, if you're interested in smaller rock shows, I can't recommend Oh My Rockness strongly enough.