Buzzword, the word processor that Adobe bought earlier this year has taken off the invitation code requirements and opened up the doors. I noticed that more of my Twitter friends were talking about it but hadn't realized that the invitation codes had been blown away.
Amit Agarwal has the first review I've seen in a while. Unfortunately Buzzword doesn't support exporting to PDF or Flash at this point. I'm not entirely sure why that is but I think they're working on some kind of integration with Share, so the PDF export resources may have been dedicated to that. So far the response to new users seems to be good. This was a product with a lot of hype and for the most part I think it's living up to it. I don't know how many people are switching away from Google Docs, but I understand that the demand for invitations was quite high. More info from both Download Squad and Lifehacker.