The Cross Platform Game Engine

Godot Game Engine

The Atomic Game Engine: http://www.AtomicGameEngine.com

a 2D Game Development Framework

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

One framework for creating powerful cross-platform games.

Xenko Game Engine

MIT Licensed Open Source version of Torque 3D from GarageGames

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

A powerful free cross-platform RTS engine

cocos2d-x for C++

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

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

The Panda3D free open-source cross-platform game engine with excellent Python support

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

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

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

Desktop/Android/HTML5/iOS Java game development framework

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

Superpowers — Extensible HTML5 app for real-time collaborative projects