I love optical illusions, but I'm a bit sick of the "which line is longest" and "hey cool, it looks like it's spinning!" ones.
I have just stumbled across (cheers, Gizmodo) a hole new class of coolness. Most 3D illusions rely upon feeding each eye a different image, through colour filters or one feed direct to each eye through lenses, or something like this. These images demonstrates that it's possible to get a 3D effect by throwing the two images at both eyes, but alternating them and confusing the brain into thinking that it's seeing an object with depth.
Joshua Heineman (lala.cursivebuildings.com) has simply taken the photo sets used in old stereogram goggles and built an animation which rapidly flicks between the images. The technique is not unique or even particularly new, but I think there needs to be more of it.
Unfortunately, the migraine you're liable to get while watching this would make the creation of a full 3D movie out of the question.