- Mobiledoc - https://github.com/bustle/mobiledoc-kit - framework and toolkit to build editors with a standardized JSON structure
- Froala - https://www.froala.com/wysiwyg-editor - best editor (IMO) for traditional HTML structure
- CKEditor (v5) - https://ckeditor.com/ - long history, very customizable, new architecture with built-in collaboration
- 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
- ProseMirror - http://prosemirror.net/ - supports collaborative editing, offers similar options to Mobiledoc for data structure
- Squire - https://github.com/neilj/Squire - used in FastMail webmail, better for text than 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, similar to CKEditor
- 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/
https://github.com/share/sharedb has some great development on real-time collaborative editing that can be used to power additional features using these editors.