Today there in an article on php magazine about a project called Yawiki PHP. This project is a rather powerful wiki system for anyone who's looking for something decent.

Yawiki is a powerful Wiki system for collaborative document generation. It supports both anonymous and authenticated users. It uses a WikiMap page to generate a hierarchy of pages and navigational elements. It supports page-level access control lists. There is an optional comment system built-in.

They've just released a new version it seems, so head on over to thier project page or read the article at phpmag.net.