AOL Instant Messenger. It's really fun, and it's free.
Do you hop on IRC at all? It's...sort of like that...except different...scratch that LOL. Here's the scoop: you talk to an individual person in a little chat window...you can hold 20 conversations at one time if you want...but I can barely keep up with 4. They have chat rooms, too. The chat rooms are not moderated...any Joe Schmoe can create a room, but it's fun if you're talking to a bunch of friends online and you guys create a room for yourselves.
http://www.aim.com
