A mãe de todos os Wikis. É a ferramenta usada na wikipedia.org. Costumava ser difícil de instalar, mas agora há um processo quase automático.

Eu uso no sempre que preciso de um wiki.

Site oficial
wikipedia.sourceforge.net

"You may wonder if you should use MediaWiki for your own wiki.. This depends on what kind of wiki you want to run. Wikipedia's software has many useful features that grew out of the needs of the users and the ideas of the developers -- but features also mean complexity. The functionality can be overwhelming, even though we try to do everything we can to keep things easy to use. Complexity also means that there are lots of places where you may have to customize the script for your own use..

Before you decide to use the Wikipedia script, please do take a look at other Wiki Engines and see if one of them might meet your requirements better. For small wikis, UseMod (http://www.usemod.com/cgi-bin/wiki.pl) is always a safe bet -- it supports [[free links]] (as opposed to CamelCase) and does not need (or support) a database. This is, of course, also a bottleneck in terms of functionality."

Vc pode/deve tentar usar os sistemas instalados no opensourcecms.com. Lá tem muito mais opções do que aqui. :)