MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
Pmwiki - PmWiki is a wiki-based system for collaborative creation and maintenance of websites.
MDwiki - Static single file CMS/Wiki done purely with client-side Javascript and HTML5.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
PhpWiki - PhpWiki is an all-in-one web-based application that was used as a clone of WikiWiki web but now has been used to edit and format books for publication.
WikkaWiki - WikkaWiki is a flexible, standards-compliant and lightweight wiki engine written in PHP.