CKEditor - Real-time collaborative future-ready rich text editor
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
Textpattern - Textpattern is an elegant content management system that is free, open source software.
Froala Editor - Froala Editor is a WYSIWYG HTML editor that enables rich text editing capabilities for the applications.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more