Elaborate is a free wiki engine written in PHP and MySQL available under the GPL

Wikipedia's underlying open source wiki engine.

Open Source Wiki engine written in PHP for the Midgard Content Management System

Feature-rich implementation with support for various databases.

A simple-to-install system which allows password protection and organization of pages into groups (GPL).

A small PHP script, using a textfile database, with a nonrestrictive license.

Implementation with MySQL backend for use on sites hosted by SourceForge.

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.

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 rights (ACLs), page comments.

Wikepage is an easy to use and small dual-mode wiki engine that can act as wiki sites and personal homepages.

Open source wiki, based on Wakka Wiki, nhanced (XHTML 1.0 and CSS compatibility) and translated into French.

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.

A PHP script that runs wiki sites.