Campfire by 37Signals
Now this is something that gets me fired up ! Finally a messaging app that you fully own and operate as you want, and host on your own server ! And it's a single on off payment !
From the guys that brought us basecamp and hey email ! Can't wait to try it out. I do have to find a purpose to use it for though.
Super simple group chat, without a subscription. And you get the code, too.