wxWidgets - wxWidgets: Cross-Platform GUI Library
CKEditor - Real-time collaborative future-ready rich text editor
Electron - Build cross platform desktop apps with web technologies
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.