wxWidgets - wxWidgets: Cross-Platform GUI Library
Notix.co - NOTIX is an audience re-engagement service based on web and in-app push notifications that work for both desktop and mobile devices.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Electron - Build cross platform desktop apps with web technologies
Appcelerator - Native apps, Mobile APIs, Real-time analytics, One Platform.