MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.

Tiki Wiki CMS Groupware - Tiki Wiki CMS Groupware, also known as Tiki (or TikiWiki), is a full-featured, web-based, multilingual, all-in-one Free/Libre Open Source Software solution. Tiki can be used to create all kinds of web apps, portals, knowledge bases, intranets, etc.

SlimWiki - Beautiful Wikis for Teams. Get started today, free forever!

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.

WikkaWiki - WikkaWiki is a flexible, standards-compliant and lightweight wiki engine written in PHP.