Holla is an open source group chat application where you can create channels, invite friends to those channels, chat and share files.
It has a totally Ajaxed interface on every level (messaging, uploading files, etc.).
A nice feature is that the file uploads can be done via drag'n drops and every user in the chat can see the upload progress bar.
Demo
http://u.bb/1GW
Info
http://u.bb/1GV
Download
http://u.bb/1GX