Piknic Électronik is a great electronic music event in Montreal that happens on (just about) every Sunday afternoon in the summer. It typically takes place in Parc Jean Drapeau, the site of the Expo 67 World's Fair, under the Alexander Calder statue "L'Homme", from about 1PM to about 9PM. It usually costs between $5-10.
There are also a few events during the rest of the year, such as a great party in the snow in January/February.