- Mobiledoc - https://github.com/bustle/mobiledoc-kit - framework to build editors with a standardized JSON structure
- Substance - http://substance.io - library for building editors and publishing systems
- ShareDB - https://github.com/share/sharedb - framework to sync any JSON document using operational transforms, add real-time collaborative editing to anything else
These use separate document structures instead of HTML, some are more modular libraries than full editors
- ProseMirror - http://prosemirror.net/ - supports collaborative editing, offers similar options to Mobiledoc for data structure
- Slate - https://github.com/ianstormtaylor/slate - framework to build more customized editors inspired by Draft, ProseMirror, Quill
- Draft - https://draftjs.org - rich text editor framework for React
- Quill - http://quilljs.com/
- Trix - https://github.com/basecamp/trix - by Basecamp team and used in v3
- Scribe - https://github.com/guardian/scribe - by the Guardian team
- Etherpad - http://etherpad.org/ - collaborative realtime editing framework, backend and editor
- Texture - http://substance.io/texture/ - editor built on Substance library
- Froala - https://www.froala.com/wysiwyg-editor - best editor (IMO) for traditional HTML structure
- Pell - https://github.com/jaredreich/pell - designed to be the simplest and smallest
- CKEditor (v5) - https://ckeditor.com/ - long history, very customizable, new architecture with built-in collaboration
- TinyMCE - http://www.tinymce.com/ - older editor, similar to CKEditor
- Textbox.IO - https://textbox.io/ - new from the owners of TinyMCE project
- Squire - https://github.com/neilj/Squire - used in FastMail webmail, better for text than images
- medium-editor - https://github.com/yabwe/medium-editor - clone of Medium.com's editor
- Medium.js - http://jakiestfu.github.io/Medium.js - clone of Medium.com's editor
- wysihtml - http://wysihtml.com/
- Redactor - http://imperavi.com/redactor/
- Summernote - http://summernote.org/
- ZenPen - http://www.zenpen.io/
- Trumbowyg - https://alex-d.github.io/Trumbowyg/ - html5 editor as jquery plugin