Wiki Software

Wikis are becoming more and more Content Management System light. Best represantative of a Wiki is the Wikipedia. A friend of mine brought me to the idea of having a closer look at Wikis.
The best sources of Wikisoftware are at Wikipedia (WikiSoftware) and c2.com (WikiEngines)
I will have a closer look at some PHP versions (PM Wiki, PHP Wiki, MediaWiki) now.