This bus stop ad, recently erected in Hamburg, Germany, changes whether or not someone's looking at it. Using a built-in camera with eye-tracking technology, it can tell exactly when someone is checking it out.
It uses this to get across an anti-domestic abuse message. When no one is looking, it shows a man hitting his wife. But when you look right at it, it changes to a picture of the couple looking happy and normal. A cool piece of tech used to powerful effect.