Multi Theft Auto is a game engine
Solar2D Game Engine main repository (ex Corona SDK)
A .NET ascii/ansi console engine written in C# for MonoGame and XNA
A multi core friendly rigid body physics and collision detection lib
A hand-crafted 2D game library in Go
Cross-platform 3D scene editor based on three.js, golang and mongodb
3D Game engine with editor
A project to create a blazingly fast Swift game engine
Java Game Engine
A truly extensible cross-platform game engine
Opensource serious game engine for multiplayer games
The Lambda-Engine is a cross platform game engine.
Zap Engine