Unity - The multiplatform game creation tools for everyone.
wxWidgets - wxWidgets: Cross-Platform GUI Library
Cyberix3D - Free online 3D Game Maker. Make your own 3D games online!
Electron - Build cross platform desktop apps with web technologies
Godot Engine - Feature-packed 2D and 3D open source game engine.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.