Yeah some of these are really incredible, thoguh I find it hard to trust many of them on the internet (two reasons, firstly because they might acutally be animated and the person is just having you on, secondly because it might be one of those things that screams at you while you are staring at it).
One of my favourites has to be the "Margaret Thatcher Illusion". Shown here with the face of some other guy:
It proves that we make judgements on people's facial expressions without really looknig at the context in which we see them.