Appcelerator Titanium - Appcelerator Open Source
wxWidgets - wxWidgets: Cross-Platform GUI Library
React Native - A framework for building native apps with React
Electron - Build cross platform desktop apps with web technologies
Onsen UI - HTML5 Hybrid Mobile App UI Framework - work with Angular, React, Vue, Meteor & pure JavaScript. Material & Flat design.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.