Recently, Google Docs added a collaboration feature to their web office suite. You can invite participants to edit the document with you, and Google Docs shows you what they’re changing in realtime.
Another popular document collaboration app: TitanPad.com. TitanPad allows you to collaborate on a document with others, without even signing up for an account. However, anyone can access your document (if they can guess the obscure URL of it, that is) so I’d recommend backing it up on your computer if you are editing it in multiple sessions.