Multi Theft Auto is a game engine
Magic Another Game Engine
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
A dead simple 2D game engine for Go
KorGE Game Engine. Multiplatform Kotlin Game Engine & Korlibs
A tool to easily transform Unreal Engine games into VR experiences
API (C#) for simulating certain aspects of a ZX spectrum for porting.
PyCAPGE - Python Classic Adventure Point and Click Game Engine
A conservative source port of Heretic that is identical to DOS
State Management and Multiplayer Networking for Turn-Based Games
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.
2D Game Engine
Low-Level Game Engine for Retro Platforms
scriptable game engine with built-in support for 2D graphics and sound
Gorgon Game Engine (GGE) is a C++ game engine that handles many tasks.