Godot Engine - Feature-packed 2D and 3D open source game engine.
Spine - Spine is 2D animation software designed specifically for software and game development.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Synfig Studio - Just another WordPress site
RPG Maker - Make your own PC game with RPG Maker. Our easy to use tools are simple enough for a child, and powerful enough for a developer. Try it free today!
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.