I am developing a chat system for zion.cz. So far, there's a first public version of the Zion Chat Server available, which is far from complete but has the basic functionality and seems to be working. Most of the time, anyway. ;-) Also, there's a specification of the protocol.
The software is free or open-source, whichever you prefer; more precisely, it uses the GNU General Public License.
So far, I am the sole author of Zion Chat Server. If you want to participate, you are very welcome!
Currently, there are two clients of the ZCP in development: I'm trying to make one in Java, and Vladia is coding in PHP. Neither is in a publishable state, as far as I know. :-)