DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
TiddlyWiki - a non-linear personal web notebook
Pmwiki - PmWiki is a wiki-based system for collaborative creation and maintenance of websites.
Ema Personal Wiki - Ema Personal Wiki uses Markdown to freely create and manage structured notes, like a website.
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.