Google Docs - Multiple people can collaborate on word processing, spreadsheet, and presentation slideshow documents. An email address is required for login. Read Getting Started with Google Docs.
Etherpad - Multiple people can work on the same text document simultaneously. The really nice part is that no account is required, so students don't need to worry about usernames and passwords. The word processor is real-time, meaning that you can watch on one computer as typing occurs from another. Other features include a chat room and colors to indicate who typed what.
Scriblink- Similar to Etherpad, Scriblink is a website for people to work on the same virtual whiteboard simultaneously. Users can upload images on which to draw, create graphic organizers, brainstorm ideas, work through math problems, etc. Like Etherpad, there's no registration needed so students can get right to work.