Bixi. I rave about it all the time still even though I no longer live in Montreal. Wow, I can't believe there's 460 stations (5,200 bikes) now! Amazing.
Price is still unbelievably cheap ($5.00 for 24 hours, $14.00 for 72 hours, $30.00 for a month, $55.00 for half season and $87.00 for an annual membership). They even have the option now of doing a one way for $2.95, although I have no idea why you would do that. The season goes from April to November so six months for $87.00 is a steal.
Guess which one I bought for my 5 days in Montreal? Yup, month membership for the win! Bonus that I can still use my old school BIXI key (that's lost half of it's paint job by now). Totally worth the $30.00 because I took 25 trips and rode almost 80km (I checked my Bixi stats 'cause I'm a nerd like that). Not too shabby.
It's the best way to explore Montreal. I can't recommend it enough whether you're a tourist or local. There's nothing better than taking a BIXI out to cruise at night with the wind gushing in your face.