TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
TextControl - Text Control Reporting combines the power of a reporting tool and an easy-to-use WYSIWYG word processor embeddable in .NET applications.
Web AppBuilder for ArcGIS - Create custom widgets and themes for Web AppBuilder.
FirstPage - First Page is a free web builder to design and manage the professional level of websites as you want.
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.