Hydra

RendezVous/ZeroConf-based Collaborative Writing for MacOs X. So it's kinda like IRC but with editing not just streaming.

http://hydra.globalse.org/

Now (due to legal challenges) known as SubEthaEdit http://www.codingmonkeys.de/subethaedit/news.html

Built by the CodingMonkeys, a team of students in Germany