One thing to know about Montreal is the the city lives and dies by its nocturnal activities.  That said, the city learned long ago that any late night clubgoer needs that "end of night" nourishment.  Grease (the fat , not the musical) seems to be the food of choice for the Montreal late night scene.  Just walking down St. Lawrence Boulevard near the Prince Arthur cobblestoned street will give a good idea of what the city has to offer. 

You can order from Focalaio's , ALTOS, and Zestos, but Altos has the most variety , Focalaio the best pizza and Caesar salad.

On Crescent Street you could always check out Wienstein & Gavino's for the 10$ @ 10pm menu, a great deal for late night diners.  Recently some of the best and more expensive restaurants have started serving late dinners after 10pm.  The list includes Café Ferreira downtown, Chez Lévesque in Outremont and the best Greek restaurant in the world (it now has outlets in NYC, Las Vegas and Athens) in, of course, the Greek neighbourhood around Avenue du Parc (Park Avenue).

New to the Montreal food scene is L'Atelier d'Argentine it is rooted in the history and nestled in the urban and regional landscape, genuine Argentinean cuisine is distinct, with a strong flavourful identity - easy to understand and even easier to love. Try their late night menu Buenos Aires Hours, $22.50 for 2-courses after 10:30pm.

Popular eats include Pizza Madona's   $1.50 pizza,  the only pizza joint in North America where you can bargain for the price of the pizza. 

Right across the street you can find La Belle Province; if it's burgers, hot dogs and poutines you're after, this is the place to find it. 

For people with more sophisticated culinary tastes, L'Express, a French bistro located at 3927 St-Denis, is open late  (the kitchen closes at 2 am) and is usually packed any night of the week, even at a late hour. 

The Mecca of late night fast food however can be found a little further down the street. 

Any visitor to Montreal can not leave without first trying Schwartz's, great smoked meat sandwich and luckily it's open late but not enough for clubbers that will switch to another deli across the street, The Main which is open 24 / but is not on par with Schwartz's, far from it.