Communauto is not so much a car rental as a car sharing facility. In order to use this service, you have to apply by sending copies of your drivers license and insurance papers, and sign up for a year.
Not useful for tourists, but invaluable when you live in Montreal and do not own a vehicle, since it allows you to grab a car whenever you need it and at short notice.
Much like the Bixi, Communauto cars are located throughout the city. They are mostly Toyota Yaris. Recently, a few hybrids have been added to the fleet. You reserve your car online or on the phone, walk over there, retrieve the keys from a little metal box close to the car, and off you go.
There are several different types of membership available, depending on how much you want to use the car. Prices consist of a fee per hour and per kilometer. Long distance rates are available with some membership plans. Conveniently, gas and insurance are included in the rental price.