TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
FreeTextBox - FreeTextBox is the most-used free ASP.NET WYSIWYG HTML editor featured in open source and commercial projects.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
Web AppBuilder for ArcGIS - Create custom widgets and themes for Web AppBuilder.
Edtr.io - Intuitive WYSIWYG web editor, open source & fully adaptable.
Mercury Editor - Mercury Editor comes as a Rails Engine, but if you'd like to use Mercury outside of a typical Rails application, you can download these packages and use them.