Robin Good: Titanpad is a web-based collaborative document editing tool that allows any number of people to work collaboratively and simultaneously on the same document.
Every user gets automatically a different color, there is no need to sign-up, and best of all there are infinite levels of Undo and you can have a dedicated subdomain where to host your collaborative docs.
The service is free. Find out more here: http://titanpad.com/
Via Robin Good, michel verstrepen