Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Powerful, mature open-source cross-platform game engine for Python
Cocos simplifies game creation and distribution with Cocos Creator
Cross-Platform C++ 2D/3D game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Flax Engine, multi-platform 3D game engine
Cross-platform 2D and 3D game engine with modular architecture
3D Procedural Game Engine Using OpenGL
Free game engine for development of desktop, mobile, and web games
Voxel game engine in C++ with OpenGL
Open source clone of the Age of Empires II engine
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
3D game engine with lua scripting
Multi-platform 2D and 3D game engine
3D Game engine with editor
Voxel game-creation platform with easy modding and game creation
GemRB is a portable open-source implementation of the Infinity Engine
Vivid3D is a C++ IDE/3D engine, with python scripting.
A free 3D game engine to create games and realtimes scenes for Windows
C++ lightweight cross-platform game engine
MIT Licensed Open Source version of Torque3D from GarageGames
Vivid is a modern C++ 3D engine using OpenGL4+
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI