Desktop/Android/HTML5/iOS Java game development framework

cocos2d-x for C++

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

CRYENGINE is a powerful real-time game development platform created by Crytek.

Free (as in freedom) open source clone of the Age of Empires II engine 🚀

One framework for creating powerful cross-platform games.

An open-source implementation of the Command & Conquer: Red Alert engine using .NET/Mono and OpenGL.

Turbulenz is a modular 3D and 2D game framework for making HTML5 powered games for browsers, desktops and mobile devices.

MIT Licensed Open Source version of Torque 3D from GarageGames

The Cross Platform Game Engine

Cross-platform 3D native C++ game framework for mobile and desktop games.

Cross-platform 2D and 3D game engine

A complete 3D game development suite written purely in Java.

Xenko Game Engine

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

🐙 Superpowers — Extensible HTML5 app for real-time collaborative projects

The Atomic Game Engine is a multi-platform 2D and 3D engine with a consistent API in C#, JavaScript, TypeScript, and C++ (http://www.AtomicGameEngine.com)

A powerful free cross-platform RTS engine

a 2D Game Development Framework

Powerful, mature open-source cross-platform game engine for Python and C++, developed by Disney and CMU

Real-Time Strategy game 3D engine coded in java 7

🎮 GDevelop is an open source HTML5 and native game creator requiring no coding skills.

A free and open 3D game engine written in D.

XLE -- the "hackable" 3D game rendering engine