So far, LivingSocial is the only group-buying web site I've actually tried myself. The process is pretty straightforward and it works as advertised. You can use the link to sign up and get $5 to spend for opening an account (just know that deals will surely cost more than $5).
On another note I kind of like their "365 things to do" page. Gives you some ideas to try out if you're ever bored in Montreal.
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