The Ren'Py Visual Novel Engine
3D Game engine with editor
Multi-platform Engine for Desktop, XBOX (UWP) and Mobile games
Stride (formerly Xenko), a free and open-source cross-platform C# game
Multi Theft Auto is a game engine
Free game engine for development of desktop, mobile, and web games
Cocos simplifies game creation and distribution with Cocos Creator
A completely free, open-source, 2D game engine
Cross-platform 2D and 3D game engine with modular architecture
An OpenGL RTS game engine written in C
3D game engine/framework in C, with Luajit and Python bindings now
Cross-Platform C++ 2D/3D game engine
Open source clone of the Age of Empires II engine
Voxel game-creation platform with easy modding and game creation
Flax Engine, multi-platform 3D game engine
Python MUD/MUX/MUSH/MU* development system
3D game engine with lua scripting
Multi-platform 2D and 3D game engine
Injectable LUA scripting system, SDK generator, live property editor
Vivid3D is a C++ IDE/3D engine, with python scripting.
Full featured MUD server with all the trimmings.
RTSish Game Engine
PyCAPGE - Python Classic Adventure Point and Click Game Engine
A free 3D game engine to create games and realtimes scenes for Windows
MIT Licensed Open Source version of Torque3D from GarageGames