TWiki

DokuWiki is a simple, yet versatile open-source wiki software. It's known for its ease of use, low system requirements, and built-in access control lists for managing user permissions.

XWiki is a free wiki software platform written in Java.

WikkaWiki is an open-source wiki application written in PHP.

Apache JSPWiki is an open-source Wiki engine, built around standard JEE components (Java, servlets, JSP).

MediaWiki is a free, open-source wiki engine that powers Wikipedia and many other wiki-based websites. It offers collaborative editing, version control, and extensive customization options.