For a Collaborative Experience --

download MoonEdit client at moonedit.com  -- wasn't opensource last I looked but it's fun.

DoriSmith says http://www.codingmonkeys.de/subethaedit/ looks similar.

A new one to the park:

http://packages.debian.org/unstable/net/gobby


See also [http://communitywiki.org/collaborative_editor CommunityWiki:collaborative_editor]