HaxeFlixel - Create cross-platform games easier and free. All with one codebase.
Unity - The multiplatform game creation tools for everyone.
Flixel Game Engine - Flixel is an open source game-making library that is completely free for personal or commercial use.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Pygame - Pygame is a set of Python modules designed for writing games.