에디터 오픈소스는 단순한 텍스트 편집기부터 CMS수준의 콘텐츠 작성기까지 정말 종류도 많고 다양한데 계속 나오는군요.

SummerNote: https://summernote.org - Super Simple WYSIWYG Editor on Bootstrap. 홍영택님이 만들고, 많은 한국분들이 참여중인 오픈소스 에디터
Quill: https://quilljs.com - An API Driven Rich Text Editor
Slate: https://www.slatejs.org - A completely customizable framework for building rich text editors
Trumbowyg: https://alex-d.github.io/Trumbowyg - A lightweight WYSIWYG editor
ContentTools: http://getcontenttools.com - A beautiful & small content editor
Squire: https://github.com/neilj/Squire - HTML5 rich text editor
Editor.js: https://codex.so/editor - Next generation block styled edito. JSON 으로 결과물 저장
ProseMirror: https://prosemirror.net/ - A toolkit for building rich-text editors on the web, 공동 편집가능 에디터
Gutenberg : https://wordpress.org/gutenberg/ - 워드프레스의 편집기
webiny : https://www.webiny.com/ - CMS for Serverless Web Development
GrapesJS : https://grapesjs.com/ - Web Builder Framework
Craft.js : https://github.com/prevwong/craft.js - A React Framework for building extensible drag and drop page editors
Draft.js : https://draftjs.org/ - Rich Text Editor Framework for React