(Watch on YouTube) Anamorphic illusions are flat designs that look one way from one direction and another way when you turn them. This impressive video shows what seems to be a series of common objects sitting on paper. Then they are turned to reveal the trickery. What really sells it to me is the camera zooming in and racking focus—Oh, and the cat.
Other Anamorphic illusions explains
Wikipedia on Anamorphosis
Via Best of YouTube
Book: Hidden Images: Games of Perception, Anamorphic Art, Illusion from the Renaissance to the Present