The Other Wikipedia
Whenever I hear about what a great community-edited, user-contributed site Wikipedia is, I think about the unsung site that's just as much of a success: Urban Dictionary. Where else can you find definitions for meh and cool beans and thousands of obscure sexual practices of varying degrees of debauchery?
I guess because there's a lot of vulgarities and obscenities, and because it's a commercial site, it doesn't get a lot of press. And the way that people writing definitions adopt a faux-erudite tone when writing them is a little bit annoying. But damn if Urban Dictionary isn't entertaining, and it meets a need just as neatly as Wikipedia does.