If you're interested in the local tech scene, you need to check out Notman House.
Home to a number of start-ups, they also host a lot of the local tech group meet-ups such as: Montreal.rb, ViMTL, and others. They also host hackathons periodically, and I've been to a few good ones there.
You can come during the day and chill-out/work in the main room for free, with wifi provided by Île Sans Fil.
The place is a bit run-down at the moment, but only because they're in the middle of massive renovations to improve and expand the whole place - you can see the work they're doing on the secondary building out the back. It looks like it's only going to get bigger and better in the future.