Desktop/Android/HTML5/iOS Java game development framework
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Atic Atac Spectrum Game Remake
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
Cross platform 2D game creation framework with Python bindings
Pear3DEngine is a modern and modular 3D development framework
Another OpenGL Game Framework for Windows
PixelLight is a cross-platform application framework