Desktop/Android/HTML5/iOS Java game development framework

cocos2d-x for C++

Godot Game Engine

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.

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

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

Superpowers — Extensible HTML5 app for real-time collaborative projects

A powerful free cross-platform RTS engine

Xenko Game Engine

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

a 2D Game Development Framework

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.