These were used on Bungie.net for clans. They were a great way for an entire group of people to communicate all at once without randomers and trolls and such entering the conversation, or spamming the forums and breaking rules.
I've been looking, but I can't seem to find any? Are there none? Damn, these would be great for the forums, IMO.
E.g. if me and a couple of friends I met here want to talk about XBOX-related crap, but we were (obviously) on these forums instead of on the XBOX, we could have a place where we could talk in private. Basically more-than-two PM's.