At WordCamp: Las Vegas, Shayne Sanderson talked a bit about a new plugin coming from Instinct (the makers of wp e-commerce). The WordPress Wiki plugin adds basic wiki functionality to WordPress posts on pages, using WordPress user accounts and revisions to manage the edits.
It’s not going to replace MediaWiki, but it’s a nice way to add some lightweight edit and revision features to a WordPress blog. I’ve heard that the Beer and Blog folks are looking at it for documentation purposes, where it should provide an easy interface for managing documents and provide a nice front-end for some of WordPress’ back-end features.
If you’re looking for a lightweight way to wikify some of your website, check out the plugin.