Collaborative Editing

Interesting. A number of disadvantages having your work based on their servers, though, such as a total lack of security.

http://etherpad.com/
http://db.tidbits.com/article/9869

A bit of research turns up a few other possibilities as well.

For Mac users; this has been around for a few years. I used it for my editor when it was free.
http://www.codingmonkeys.de/subethaedit/

Eclipse also has a communications framework with some interesting possibilities.
http://www.eclipse.org/ecf/
http://www-128.ibm.com/developerworks/opensource/library/os-ecl-commfwk/
http://www.infoq.com/news/2008/06/eclipse-ganymede-ecf
http://wiki.eclipse.org/DocShare_Plugin

I just purchased Coda (editor) and it has a cool collaborative feature where you can invite users to edit the same document. This seems to be becoming a popular feature. I still have not ventured into using it yet (and not sure when I will).