wxWidgets - wxWidgets: Cross-Platform GUI Library
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Electron - Build cross platform desktop apps with web technologies
Lumix Engine - Open source 3D game designer and engine written in C++.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
MOS - Lightweight game engine.