wxWidgets - wxWidgets: Cross-Platform GUI Library
PowerSchool - PowerSchool provides a K-12 education technology platform for operations, classroom, student growth, and family engagement.
Electron - Build cross platform desktop apps with web technologies
Clever - syncing between education applications for K-12 schools
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Teachable - Create and sell beautiful online courses with the platform used by the best online entrepreneurs to sell $100m+ to over 4 million students worldwide.