One of the first places I visited on my first trip to Montreal many years ago, and every time I'm in town, I still stop in.
Great beer selection and the food (all vegetarian) can't be beat. Prices are cheap, but be sure to be generous and leave a big tip to the employees, as it's a collective and they don't get paid to be there.
Really awesome shows too - the few times I've randomly popped in, the bands have impressed me enough to buy CDs and fall in love. You can also come in and buy tickets for the bigger-name shows that play at La Sala Rossa across the street.
Definitely one of the coolest places to hang out in the city!