Algodoo

Pretty much just basic magnets with air-dense pupils. Does have rotation disabled for the individual circles, though, in case you're looking for that simple postStep script.

Instructions: Simply make something on the default A collision layer, and bring it up close to the pair of eyes, and it'll shift its eyes to look at it. Second plane will catch it because, of course, if I just left it at the lone green one then the eyes would constantly be looking down.