Batflat - Lightweight, fast, easy, and free CMS.
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
nCMS - Open source enterprise Java CMS engine
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Wagtail CMS - Django-based Content Management System.
ClassicPress - The WordPress fork. No Gutenberg. Great future!