Earlier this evening I gave a presentation for the 2010 Reform Symposium. The topic of the presentation was using backchannels in your classroom. During the presentation I explored some of the problems that using backchannels can solve for teachers. The recording of the presentation should be available here soon. In the meantime, I promised that I would post the slides from the presentation. The second to last slide contains a list of tools that can be used for hosting backchannel chats.
(Yes, something weird happened when I uploaded to SlideShare to cause my first slide to get a bit off).

Here are some related items that may be of interest to you:
Neat Chat – Quickly Create an Ad-Free Chatroom
Five Platforms for Classroom Back-channel Chat
Back-channeling During a Class Viewing of Glory