if you're visiting and you need to get around Montreal, Bixi's the way to do it.
parked all over the city, you pick up a bike where you are, drop it off where you go - you don't need to return to the same spot you picked it up from (which is why it's better than zipcar). Montreal is a fantastic biking city, and as a total non-cyclist, even i hopped on a bike to trek around.
words to the wise: do not pick up a bike and keep it out. return to racks frequently, lest you be charged lots o' $$$ - see Derrick V's note about how to stretch your Bixi dollar. also, they don't come with helmets, so make sure you bring your own!