Cross-Platform C++ 2D/3D game engine
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Cocos simplifies game creation and distribution with Cocos Creator
Flax Engine, multi-platform 3D game engine
Free game engine for development of desktop, mobile, and web games
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Cross-platform 2D and 3D game engine with modular architecture
Voxel game engine in C++ with OpenGL
3D game engine with lua scripting
3D Procedural Game Engine Using OpenGL
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
3D Game engine with editor
Powerful, mature open-source cross-platform game engine for Python
Voxel game-creation platform with easy modding and game creation
Multi-platform 2D and 3D game engine
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.
3D C++ Game Engine - yet another open source game engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI