Apache JSPWiki - Apache JSPWiki is an open source Wiki engine, feature-rich and built around standard JEE components.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
MyPHP Forum - MyPHP Forum is an advanced bulletin board system written in PHP with MySQL as backbone data storage.
MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
WackoWiki - WackoWiki is a light and easy to install multilingual Wiki-engine.
Niles - A wiki you can talk to