Campfire like chat plugin?

I've got to write a little app at work for doctors and med students to view a streaming video feed, and the users want the ability to have a chat window. I've got everything else figured out, authentication, the stream, etc. The chat feature is something I don't really know where to start on. It does not need to be nearly as fancy as Campfire. The only "extra" is that the chats need to be logged transcripts can be downloaded later. Does anyone know of a plugin that might do this? Or, I'd love to hear of some ideas you may have. Thanks.

Good call. That's helpful.

Thanks, unfortunately this is for a medical school and there is the slightest chance that HIPAA type info could be chatted over. Yes, the docs know they shouldn't but they talk about patients via unencrypted email when they aren't still too. So having chats stored on an external site is a no go.