Writely is a web-based word processor (one of the pieces in Jason Kottke's WebOS) that allows you to edit documents collaboratively. Unfortunately it's currenly in private beta (anybody got an invite? Note to invitation-seekers: beta users can only invite people to work on a particular document; an invite won't let you create your own documents.), but seems to have some pretty cool features. You can upload a Word document and edit it in your web browser, invite others to edit it, roll back to previous versions of your document, publish a read-only version for the masses, and export to Word or HTML.

Update: If you want to take Writely for a spin, Marc Orchant over at The Unofficial Microsoft Weblog is handing out invites to collaboratively edit a document using Writely.