Cross-Platform C++ 2D/3D game engine
Powerful, mature open-source cross-platform game engine for Python
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
The Open Toolkit library is a fast, low-level C# wrapper for OpenGL
3D Procedural Game Engine Using OpenGL
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D
3D game engine/framework in C, with Luajit and Python bindings now
A modern, cross-platform Quake game engine based on FitzQuake.
GemRB is a portable open-source implementation of the Infinity Engine
Vivid3D is an open-source 3D engine with powerful 2D UI.
3D Scene Builder
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
Cross-platform 2D and 3D game engine
Blueshift is a cross-platform 3D game engine implemented in C++
Portable game engine with main focus on point-and-click adventures
2d/3d Game Engine