How To: Show a set of keys on screen, as well as if they are pressed or not?

So I want a program (pref free) that shows a predefined set of keys (lets say WASD), in a predesignated layout (lets say the same way as they are on the keyboard). Then when one of these keys (physical) are pressed, I want the co-responding button (on screen) to light up / change color.

Any ideas?