Via the ever-awesome Richard Wiseman comes this very trippy and distressing illusion [WARNING (seriously): if you are prone to motion sickness or visually-induced epilepsy, don't got there. You will regret it!]
I tried it, and it screwed up my eyesight for about a minute... though it was a pretty fun minute. And I have to say that IT WAS BETTER THAN CATS. I WILL SEE IT AGAIN AND AGAIN.