BuddyPress.org

@Xevo:

Andy has installed on the codex a very quick and simple way to modify the individual pages of the codex. It’s a customized setup of sorts, but it’s really just another WPMU blog that everyone has the ability to edit.

In future versions of WordPress (probably 3.0) posts will become more robust and be better suited for something like this. Until then the codex is basically just pages and sub pages with a theme that knows how to make sense of the hierarchy.