Great service and a knowledgeable staff, Fitz & Follwell is one of my favourite bike shops in Montreal. Even though they're specialty seems to be leisure bikes, they're well equipped to tune/repair any bike that rolls through the door. They also have a great selection of locks, helmets and other bike accessories/paraphernalia.
Oh, you can also rent bikes here at a very reasonable rate. And while I haven't been on one, I hear that their bike tours of the city are fantastic.
N.B. During winter months, they also offer a variety of different services (cross country ski rental, snowshoes, etc).