A new Web 2.0 tool launched in November: Etherpad. If you've ever been frustrated trying to use Google Docs--inviting all the students, wait time to see changes, not knowing who added what--Etherpad may be for you. It's in beta test right now, and it's been so popular, they've had to go into closed Beta mode. But there's a link to email and ask for an account, and it only takes them a day or two to respond.
Etherpad creates an online collaborative document with real time updating as people add information. Our middle school history teacher is using it currently to teach his students note-taking skills. Better yet, as each student signs in (they only need a URL, not an account), they are assigned a different color, so you automatically know who is writing what.
It allows you to save different versions of the project, creating a unique URL for each one.
Right now, it only supports plain text, but RTF is a possibility in the future.