TUAW has a link about Subversion for Writers.
Subversion is a popular open source version control system. “It allows you to work collaboratively with folks on the same files (in most cases code) without fear of overwriting the work of others. Subversion tracks all the changes made to those files, and who did them, and allows you to rollback changes or branch off into different directions with having to worry about mucking up the entire project.”
The instructions are a little hairy if you’re not used to the Terminal but you can always get someone to help you set it up and host it for you.
(Now, wouldn’t it be neat if you could find a collaborative text editor (like SubEthaEdit) which also had automagical Subversion built in?)