HaxeFlixel - Create cross-platform games easier and free. All with one codebase.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Unity - The multiplatform game creation tools for everyone.
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!
OpenFL - Build games and applications for almost every platform imaginable -- Windows, Mac, Linux, iOS...
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.