Writely - The Web Word Processor
Although I am a big fan of the Backpack suite of social software, I wasn't overly impressed with Writeboard. Here is an alternative that looks even better. On the one hand, it's been acquired by Google (usually a good sign), although on the other hand, this means no registrations for now. I'd really like to try this out. In the meantime, I submitted my email address to be notified when it is open again.
At first glance, the main difference seems to be in the formatting options and the export/import flexibility. Hopefully, with collaborative software being the next "wiki" more sites like this will pop up and we'll have dozens to choose from.