MediaWiki
   Wikipedia's underlying open source wiki engine.
   http://www.mediawiki.org/

PHPWiki
   Feature-rich implementation with support for various databases.
   http://phpwiki.sourceforge.net

Particle Wiki
   A light PHP/MySQL wiki engine with archived history, search, admin panel and skins (released under proprietary Particle Soft End User Licence).
   http://www.particlesoft.net/particlewiki/

Tiki
   A kitchen-sink content management system that combines wiki with a Slashdot-style article system, blogs, forums, image galleries, chat, and so on. Pear:DB backend, LGPL.
   http://tikiwiki.org/

Wacko Wiki
   Small, lightweight, handy, expandable, multilingual Wiki-engine based on Wakka Wiki. WYSIWYG editor, easy installer, many localizations, email notification on changes/comments, several cache levels, design themes (skins) support, XHTML compliance, page ri
   http://www.wackowiki.com/

Wikepage
   Wikepage is an easy to use and small dual-mode wiki engine that can act as wiki sites and personal homepages.
   http://www.cyrocom.com/

Wikini
   Open source wiki, based on Wakka Wiki, nhanced (XHTML 1.0 and CSS compatibility) and translated into French.
   http://www.wikini.net/

WikkaWiki
   A free PHP/MySQL wiki engine based on forked code from WakkaWiki, with a number of features beyond the original Wakka, including a GUI page editor, advanced syntax highlighting and mindmap support. Released under GPL.
   http://wikkawiki.org

Wikki Tikki Tavi
   A PHP script that runs wiki sites.
   http://tavi.sourceforge.net/

net.nemein.wiki
   Open source wiki engine written in PHP for the Midgard Content Management System.
   http://www.midgard-project.org/documentation/net-nemein-wiki/

sfWiki -- a Wiki for SourceForge sites
   Implementation with MySQL backend for use on sites hosted by SourceForge.
   http://sfwiki.sourceforge.net/