DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Wiki.js - An open source, modern and powerful wiki app built on Node.js, Git, and Markdown.
Textpattern - Textpattern is an elegant content management system that is free, open source software.
TiddlyWiki - a non-linear personal web notebook
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more