Open-source engine for Heroes of Might and Magic III
Multi Theft Auto is a game engine
Zig game engine & graphics toolkit
Free game engine for development of desktop, mobile, and web games
Cross-Platform C++ 2D/3D game engine
The Open Toolkit library is a fast, low-level C# wrapper for OpenGL
A pure Go game engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Go 3D Game Engine
A space race game in the style of A Dark Room
Gomoku Game: Playing with Computer
A 2D Java game engine