Get it and start using it right away for free! The Citrus engine is an open-source ActionScript 3 based community project that you can extend or alter in any way to fit your needs.
2D & 3D physics at your service
Box2D, NAPE and AwayPhysics are available out of the box. With Citrus you have access to three supported physics engines enabling you to create 2D or 3D experiences. You can even build a 3D game that uses 2D physics, like a side-scroller platformer.