A pretty basic rideshare listing page.
MoreMontreal provides a rideshare service where you can post and search for rideshares to and from Montreal.
Carpooling / rideshare site for Montreal. There are some long distance rideshares available.
HitTheRoad.ca is a rideshare web site that focuses on rides between Toronto and Montreal (and back) in rental cars. It currently costs $35 each way.
AmigoExpress is a rideshare web site in Canada, but it's in French! I think it's focused on Quebec and other French speaking parts of Canada.