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.
Pmwiki - PmWiki is a wiki-based system for collaborative creation and maintenance of websites.
DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
PukiWiki - PukiWiki is a functional and reliable wiki software that has been widely using for Japanese wikis and is using the PHP framework.
MDwiki - Static single file CMS/Wiki done purely with client-side Javascript and HTML5.