During the period of Prohibition in the United States, Montreal became well known as one of North America's "sin cities" with unparalleled nightlife, a reputation it still holds today. In part, its bustling nightlife is attributed to its relatively late "last call" (3 a.m.), a large university population, the drinking age of 18, and the excellent public transportation system combines with other aspects of the Montreal culture to make the city's night life unique. The diversity of the clubs in Montreal attests to the popularity of its night life, with night clubs, pubs, bars and singing bars ("boîte à chanson"), Latin clubs, African clubs, jazz clubs, lounges, after-hours houses, and strip clubs all attracting different types of customers.
The most active parts during Montreal night life are the Downtown and the Quartier Latin. Saint-Denis street, which goes across the Quartier Latin, attracts a majority of the French-speaking population. Saint-Laurent Street (known locally as "the Main") is also one of the most popular streets. A majority of English-speaking Montrealers frequent the western part of the Downtown, with Crescent Street being one of the most popular streets in this sector. These three streets are all crossed by Downtown's most commercial street, Sainte-Catherine Street, which extends to its East in the heart of Montreal gay night life.
About Montreal Nightlife from a Montrealian:
Both the drinking and clubbing ages are 18 in Montreal. Though there are a few places (i.e. Tokyo Bar) that up the age to 21 to attract the "older" crowd. There are two streets that hold the bulk of Montreal's nightlife. One is Crescent and the other is St. Laurent. You should check out Karina which is part of the Sir Winston Churchill Complex on Crescent. And you should check out McKibbins which is an awesome Irish pub on Rue Bishop (one block west of Crescent) with a dance floor on the second floor (no cover charge but mandatory coat check).
Hope you have fun clubbing in Montreal!