GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
CKEditor - Real-time collaborative future-ready rich text editor
Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
Summernote - Summernote is a JavaScript library that helps users create WYSIWYG editors online.
JavaFX - JavaFX is the next step in the evolution of Java as a rich client platform.
Froala Editor - Froala Editor is a WYSIWYG HTML editor that enables rich text editing capabilities for the applications.