Froala Editor!

TinyMCE is a platform independent web based Javascript HTML WYSIWYG editor control released as Open Source under LGPL by Moxiecode Systems AB. It has the ability to convert HTML TEXTAREA fields or other HTML elements to editor instances. TinyMCE is very easy to integrate into other CMS systems.

CKEditor is an open source WYSIWYG rich text editor and collaboration solution designed to bring common word processor features directly to web pages, simplifying their content creation.

Alloy Editor is a modern WYSIWYG editor built on top of CKEditor, designed to create modern and gorgeous web content.

WYSIWYG Editor built with jQuery, Bootstrap and Font-Awesome. MIT-licensed. 2500+ stars on GitHub.

Trumbowyg is a very small jQuery plugin (16 kB minified) for creating a powerful WYSIWYG editor. Features:

CLEditor is an open source jQuery plugin which provides a lightweight, full featured, cross browser, extensible, WYSIWYG HTML editor that can be easily added into any web site.

Aloha Editor is an HTML5 WYSIWYG Editor that gives you a total new experience when editing. It's faster than existing WYSIWYG technologies and offers unprecedented WYSIWYG functionalities.

MediumEditor is a Medium.com WYSIWYG editor clone, a dead simple inline editor toolbar. Uses contenteditable API to implement a rich text solution.

Redactor Text Editor is a WYSIWYG html editor.Redactor provides smooth and clean design, limitless flexibility and extensibility, simple and super-efficient API and amazing user experience.

NicEdit is a free inline wysiwg html editor you can easily add to your site in seconds! NicEdit is a Lightweight, Cross Platform, Inline Content Editor to allow easy editing of web site content on the fly in the browser.

Finally, a free open source cross-browser WYSIWYG editor thats packed with every rich-text editing feature you need.

HotEditor is a browser-based rich text editor that provides an advantage in simplifying the workload and providing more options. HotEditor can convert HTML to BBCode and BBCode to HTML (great idea for integrating Hoteditor to forum software).