Commented and reported by Brandon De Hoyos, Instant Messaging Guide of About.com

Groups.im -- Hot Chat Rooms on Groups.im

The service, which launched in 2008, works by placing the chat room groups of choice on your IM Buddy List. By double-clicking and sending an IM to that group, Groups.im automatically connects you, regardless of IM client, to up to 600 members at one time in that chat.