Slate is a completely customizable framework for building rich text editors.

Slate lets you build rich, intuitive editors like those in Medium, Dropbox Paper or Canvas—which are becoming table stakes for applications on the web—without your codebase getting mired in complexity.

an open source rich text editor based on HTML5 technology and the progressive-enhancement approach.

Aloha Editor is an open source WYSIWYG editor that can be used in webpages. Aloha Editor aims to be easy to use and fast in editing, and allows advanced inline editing.

CKEditor is an open source WYSIWYG text editor designed to bring common word processor features directly to web pages, simplifying their content creation. Its core code is written in JavaScript.

Xinha is a powerful WYSIWYG HTML editor component that works in all current browsers. Its configurabilty and extensibility make it easy to build just the right editor for multiple purposes, from a restricted mini-editor for one database field to a full-fledged website editor.