The Socialtext team have upgraded joiwiki. It is still requires a username to edit, but we've added the ability to register online so you don't have to email us for an account. Fee free use it for anything that isn't illegal or spammy. Please note that the Creative Commons license for joiwiki is Attribution 2.0. The upgrade also includes the ability to integrate Technorati Cosmos and RSS feeds into pages. (Click edit and look at the source on either of the linked pages to see how you can do it for different URLs.) The edit window has also been upgraded.
Disclosure: I'm on the board of Socialtext