Article on Synchronous Writing:
Authentic Classroom Experiences: Anonymity, Mystery, and Improvisation in Synchronous Writing Environments by Carolyn Knox, Ph.D.
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Subethaedit
Formerly called Hydra, this is a Mac OS X application that is a Rendezvous based, collaborative text editor that allows several users to simultaneously work on text documents. User can either be in proximity to each other or can connect from anywhere in the world using an IP address.
www.codingmonkeys.de/subethaedit/

Wiki
Wiki is a composition system, a discussion medium, a repository, a mail system, and a chat room; it's a tool for collaboration
http://c2.com/cgi/wiki