TinyChat is amazing, amazing, amazing. It's a web-based chat platform that doesn't require a sign-in. This means that if your friends don't all have AIM or gchat or yahoo! or whatever, you can still all chat in a room together! One person creates a room and then sends the url to whoever he/she wants to invite to the room. Of course, being the forward technology minds that the creators are, they have also provided links so you can share it to FaceBook, Twitter, and MySpace.
I just tried it out with some gals from my EMP class and they loved it. It was really simple and straight-forward. I can't believe how easy it was to get everyone into one room like that.