ClassicPress - The WordPress fork. No Gutenberg. Great future!
Concrete CMS - Concrete CMS is an open source content management system for teams. A website builder with built in tools make editing content easy.
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Mezzanine - An open source content management platform built using the Django framework.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Automne - Automne is an all-in-one free and open-source content management system that comes with the reliability to manage thousands of pages.