Landmark Cinemas usually has at least 2-3 contests going at once. At the moment, they have a whooping five! The prizes are themed according to new theatrical releases and often feature prepaid visa cards, movie tickets, airfare/tickets to premieres, blu-rays (if you still collect those) and clothing.
Cineplex offers similar contests. Ticket prizes are redeemable at Cineplex theatres.
Tribute Magazine also has several contests running at once. My favourite is their "Win Movies for a Year" contest they run annually!
Scene Creek specializes in interview and reviews pertaining to entertainment and films of interest to Canadians. They also offer a variety of contests for movie tickets and prescreening shows.
Warner Bros also provides contests for their movies.