- Froala - https://www.froala.com/wysiwyg-editor - solid editor, most flexible and feature filled
- Trix - https://github.com/basecamp/trix - by Basecamp team and used in v3
- Scribe - https://github.com/guardian/scribe - by the Guardian team
- Draft - https://jpuri.github.io/react-draft-wysiwyg - built on ReactJS
- Slate - https://github.com/ianstormtaylor/slate - framework to build more customized editors
- Squire - https://github.com/neilj/Squire - used in FastMail webmail, better for text than images
- CKEditor - http://ckeditor.com/ - solid and very customizable, lots of plugins, now dated in look/feel
- ProseMirror - http://prosemirror.net/ - real-time collaboration features, not great with images
- Quill - http://quilljs.com/
- Summernote - http://summernote.org/
- wysihtml - http://wysihtml.com/
- Etherpad - http://etherpad.org/ - collaborative realtime editing
- TinyMCE - http://www.tinymce.com/ - older editor but still good
- Textbox.IO - https://textbox.io/ - new from the owners of tinymce project
- Redactor - http://imperavi.com/redactor/
- ZenPen - http://www.zenpen.io/ - very minimalist, best for text
- Medium.js - http://jakiestfu.github.io/Medium.js
- medium-editor - https://github.com/yabwe/medium-editor
- Trumbowyg - https://alex-d.github.io/Trumbowyg/
I also want to point out https://github.com/share/sharedb which has some great development on real-time collaboration and editing of data that can be used to power additional collaboration features using these editors.