CKEditor - Real-time collaborative future-ready rich text editor
Penflip - Typewrite is a real-time collaborative writing environment with Markdown support, versioning and Dropbox syncing. Alternatives to Penflip for Web, Windows, Mac, Linux, iPhone and more.
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
Cleartext - A text editor that allows only the 1,000 most common words
Quill - Powerful, API-driven rich text editor
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.