Open source clone of the Age of Empires II engine
fheroes2 is a recreation of Heroes of Might and Magic II game engine
Powerful, mature open-source cross-platform game engine for Python
3D game engine with lua scripting
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
Vivid3D is a C++ IDE/3D engine, with python scripting.
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.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
KlayGE is a cross-platform open source game engine
The second demo of my game engine.
A Simple Game Engine
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Open source, moddable game engine for the "Polygon Love 2" game
The Lambda-Engine is a cross platform game engine.