Godot Engine – Multi-platform 2D and 3D game engine
Frameworks for building games across multiple platforms.
Turbulenz is a modular 3D and 2D game framework for making HTML5 powered games for browsers, desktops and mobile devices.
A completely free, open-source, 2D game engine built on proven torque technology.
A powerful free cross-platform RTS game engine. - Report issues at https://springrts.com/mantis/
Cocos2d-x is a suite of open-source, cross-platform, game-development tools used by millions of developers all over the world.
The Cross Platform Game Engine
Open-source, cross-platform, C++ game engine for creating 2D/3D games.
A complete 3-D game development suite written in Java.
Free (as in freedom) open source clone of the Age of Empires II engine
One framework for creating powerful cross-platform games.
Desktop/Android/HTML5/iOS Java game development framework
Cross-platform 2D and 3D game engine
Real-Time Strategy game 3D engine coded in pure java
Phaser is a fun, free and fast 2D game framework for making HTML5 games for desktop and mobile web browsers, supporting Canvas and WebGL rendering.