What is a good web based, group collaboration/chat tool?

I’m looking for some sort of collaboration tool that is based online, is free, and can manage several people at once.

I’m using it for a large group of people (20–30) to all talk to each other live. Document manipulation/access would be a bonus, but my main concern is simply allowing everyone to talk to each other in a large contained chat area.

One option I’ve found is disposable chat rooms, but are there any similar options that are a bit more organizable and adept to handling several people and several conversations at once?

Any suggestions?