Cross-Platform C++ 2D/3D game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
Powerful, mature open-source cross-platform game engine for Python
3D Procedural Game Engine Using OpenGL
3D game engine/framework in C, with Luajit and Python bindings now
Flax Engine, multi-platform 3D game engine
The Open Toolkit library is a fast, low-level C# wrapper for OpenGL
A modern, cross-platform Quake game engine based on FitzQuake.
GemRB is a portable open-source implementation of the Infinity Engine
A free 3D game engine to create games and realtimes scenes for Windows
An isometric arcade-style game of skill, puzzles and exploration
3D Scene Builder
Vivid3D is an open-source 3D engine with powerful 2D UI.
C++ lightweight cross-platform game engine
Vivid is a modern C++ 3D engine using OpenGL4+
Go 3D Game Engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
3D Game engine with editor
A 2D game engine and level editor
Cross-platform 2D and 3D game engine
Opensource Game Studio