Sinapse Lite is an ultra-small wiki package that uses MySQL as a backend.
This powerful system allows for users to modify existing posts or create new ones, with a unique BB-code-esque system, such as using to mark a location and Go here to go to the named mark, and also allows for linking to other topics by encasing them in brackets.
There is also a great revision system that compares changes to previous revisions by percent so you can tell if someone had abused the topic and replaced more than they should have. It is possible to load these revisions.