I recently wrote about the potential of Google Docs and Zoho Writer als online writing tools for scientists. Buzzword is another web-based writing tool and last week was opened for new users. Buzzword is different, because it is based on Adobe Flash, which makes for a much nicer user interface.
As expected, Buzzword doesn't handle references as flexible as Microsoft Word. But you can at least get similar functionality with endnotes. I expect many small changes before version 1.0, most importantly support for the Symbol font.
You should give Buzzword a try if online document sharing with a slick user interface is important to you.