An odd and interesting example, in that it's only sort of collaborative, and only sort of about communication. It's aimed primarily at children, although there are significant numbers of adult participants. Its CommonContext is a fictional / fantasy world of cute talking creatures, "faeries" of various sorts, prehistoric deserts, snow-covered mountains, and lots of shops; it is definitely a community (rather than a simple game site), in that it has many prominent features for direct and indirect interaction between members.
It was apparently launched in 1999 by a couple of college kids, and has since grown into a humungous operation, with physical merchandise you can buy, and innumerable fan sites, and even a [magazine]. It requires registration and login (LoginsAreEvil) to see much beyond the front page, but it's free. The advertising is pervasive. There are mysteries to why it works so well; obviously kids turn out to like cute online animals, but what more general lessons about community building can we learn from it?
References (something more recent would be a nice addition):