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
Free game engine for development of desktop, mobile, and web games
Multi Theft Auto is a game engine
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
Multi-platform 2D and 3D game engine
Injectable LUA scripting system, SDK generator, live property editor
Full featured MUD server with all the trimmings.
PyCAPGE - Python Classic Adventure Point and Click Game Engine
MIT Licensed Open Source version of Torque3D from GarageGames
Open source reimplementation of GTA San Andreas game engine in Unity
Opensource crossplatform Fallout 2 game engine writen in C++ and SDL
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
2D Game Engine
2D Game Engine with Lua Scripting made on top of SFML