Rex Engine is a 2D game engine for Unity! It gives you crunchy, precise platformer physics and a huge library of classic game mechanics like running, jumping, and shooting right out of the box. It's designed to save you time and get you up and running on your Unity game as quickly as possible!
Buy it on the Unity Asset Store: http://u3d.as/RLr
Official Rex Engine site: http://www.skytyrannosaur.com
