Monkey X!

Godot is a fully featured, open source, MIT licensed game engine. It focuses on having great tools, and a visual oriented workflow that can export to PC, Mobile and Web platforms. It is a cross-platform 2D and 3D game engine and game development IDE.

Scirra Construct is a 2D game development engine with a focus on building games visually. No programming is required though the software uses a visual method for event handling that relies on programming concepts like variables, loops and conditions. It provides a powerful form of scripting that's as easy as clicking, dragging and dropping.

QICI Engine is a free and opensource JavaScript library, based on free and opensource game engine Phaser, with a web-based comprehensive suite of HTML5 game making editor.

Amazon Lumberyard is a free, cross-platform, 3D game engine for you to create the highest-quality games, connect your games to the vast compute and storage of the AWS Cloud, and engage fans on Twitch.By starting game projects with Lumberyard, you can spend more of your time creating great gameplay and building communities of fans, and less time on the undifferentiated heavy lifting of building a game engine and managing server infrastructure.

Pygame is a set of Python modules designed for writing games. Pygame adds functionality on top of the excellent SDL library.

The Atomic Game Engine features a cross-platform editor with deployment to Windows, OSX, Linux, WebGL, Android, and iOS. Applications can be built using JavaScript, TypeScript, native C++, and C# scripting is in development. The Atomic Game Engine is a full source solution under the permissive MIT License on GitHub

Construct Classic is a free, DirectX 9 game creator for Windows, designed for 2D games. It uses an event based system for defining how the game behaves, in a visual, human-readable way - you dont need to program or script anything at all. Its intuitive for beginners, but powerful enough for advanced users to work without hindrance.

Duality is a modular 2D game engine / editor combo, written entirely in C# and built on top of an OpenGL backend. It's open source, features an open architecture that invites extensibility and technical ownership, and it embraces C# as a language rather than a scripting solution.

Game Editor is the open source game design software that gives you the power to create the games of your dreams, and, unlike other game creation tools, gives you the chance to get and change the source code of the game creator and design and develop 2D games for iPhone, iPad, Mac OS X, Windows, Linux, Pocket PC, Windows Mobile, Windows Mobile-based Smartphones and GP2X.

Script runtime for rapid software development.

Cocos2d-x is an open-source and cross platform open source free 2D game engine for mobile game development known for its speed, stability, and easy of use

The WorldForge project is the original open-source MMORPG project, dating back to 1997. The intent lies in creating a widely used development framework and set of libraries by motivating interested developers to improve on the original code.

Whitestorm.js is a framework for 3D web apps built with Three.js technology. It implements a core for fast development of a 3D scene with physics.