Cocos2d-x is a suite of open-source, cross-platform, game-development tools utilized by millions of developers across the globe. Its core has evolved to serve as the foundation for Cocos Creator 1.…

Godot Engine – Multi-platform 2D and 3D game engine

UPBGE, the best integrated game engine in Blender

MIT Licensed Open Source version of Torque 3D from GarageGames

MIT Licensed Open Source version of Torque 2D game engine from GarageGames

SDL2# - C# Wrapper for SDL2

FNA3D - 3D Graphics Library for FNA

Simple Directmedia Layer

FNA - Accuracy-focused XNA4 reimplementation for open platforms

Theorafile - Ogg Theora Video Decoder Library

FAudio - Accuracy-focused XAudio reimplementation for open platforms

Giant stub of the Xbox Live features in XNA4

XNA4 Xbox Live reimplementation for Steamworks

FNA's XNA4 Specification Mismatch Test Program

LÖVE is an awesome 2D game framework for Lua.

A completely free, open-source, 2D game engine built on proven torque technology.

Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D engine that enables developers and content creators to build AAA games, cinema-quality 3D worlds, and high-fidelity simulations wit…

CRYENGINE source code.

Bullet Physics SDK: real-time collision detection and multi-physics simulation for VR, games, visual effects, robotics, machine learning etc.