Zwiki is a GPL'd WikiWikiClone based on the powerful ZopeApplicationServer (Zope), from which it gains a great deal of leverage. Zwiki is flexible and has some unique capabilities, combined into a reasonably intuitive whole. It combines well with other Zope add-ons such as PloNe. The polish and documentation is improving. We'd welcome your help.
Zope is cross platform and easy to install or find free hosting for; if you can get over the Zope requirement, in my biased opinion you should take a close look at Zwiki. (And if it's not right for you, I'd love to hear why - thanks!)