Voxel game engine in C++ with OpenGL
Cross-Platform C++ 2D/3D game engine
Free game engine for development of desktop, mobile, and web games
GemRB is a portable open-source implementation of the Infinity Engine
Mini game engine for games104
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Paintown is a 2d fighting game engine.
A roguelike game engine/editor
WGFX 2D Game Engine
A DirectX based graphics/physics engine
A simple WinAPI game engine