Multi Theft Auto is a game engine
A dead simple 2D game engine for Go
Magic Another Game Engine
3D game engine with lua scripting
A tool to easily transform Unreal Engine games into VR experiences
A conservative source port of Heretic that is identical to DOS
Open source reimplementation of GTA San Andreas game engine in Unity
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Low-Level Game Engine for Retro Platforms
scriptable game engine with built-in support for 2D graphics and sound