Queue de Cochon on Plaza St-Hubert make some great sausage. Their boudin aux pommes is to die for. I don't like William J. Walter so much although I do enjoy their knackwurst. Their sausages are often too lean and all have the same texture. There's a place on De Rouen in Hochelaga called Saucisses Polonaises. They make the sausages sold at Wawel. I like what they do, but I'm really no expert on Polish sausage.
Saucisses Polonaises Inc
2657 rue de Rouen
La Queue de Cochon Inc
6400 rue Saint-Hubert
You could probably put together a selection of passable sausages at the Jean Talon Market. Start at Cochons Toutes Rondes, drop by one of the Maghrebian butchers for merguez, score some game sausages at Prince Noir or Les Volailles et Gibiers du Marché, wander over to Milano for some of their decent Italian links, check out the offerings at Boucherie du Marché and <snore> Walter, etc.
In Côte-des-Neiges, Boucherie Atlantique has a wide selection of German sausagues, probably the best of their kind in the city (not saying much). Also in the same 'hood are merguez at the various north African butchers (Farhat is tops) and Boucherie de Paris (merguez is good, chorizo pretty good, others meh). Germans I know say that the problem with BA's sausages is that, due to choice or government regulations, they omit certain animal parts key to true deliciousness. The main problem with BDP's -- and those of many other purveyors -- is that they have cut back on fat and salt in deference to "les goûts modernes" (as one butcher put it). Other butchers over-process.
On the whole, Montreal's not a great sausage town, and more's the pity.