This video is from a game where you chase around "invisible" goblins. The only way to find these goblins is to shine a flashlight on them and follow their shadows. Once you track a goblin down, you use a vacuum cleaner to remove it (of course).
As silly as this sounds, it is very well done. The flashlight takes a computer generated 3D goblin and figures out how to orient it and then removes light from the flashlight so that it creates a shadow that looks like it belongs to a little goblin. The flashlight can be oriented in anyway, and so the shadow has to be constantly recalculated.
When you watch this game, you forget about all the technical issues and just see shadows of goblins running around. Well done! Here's the SIGGRAPH writeup.