Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
CKEditor - Real-time collaborative future-ready rich text editor
ClassicPress - The WordPress fork. No Gutenberg. Great future!
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.