The Right Brain vs. Left Brain Test (PerthNow)
This is one of the coolest optical illusions I have seen in a while. I challenge you to find ways to trick your brain into seeing her spin in the opposite direction. I've had the most success at this by scrolling down in my web browser so that I only see her foot, convincing myself that her foot is spinning in the opposite direction from what I naturally see, and then scrolling back up.
Yale neurologist Steven Novella has a good explanation for how this illusion works. He also completely debunks the claim that this is a "right brain vs. left brain" test (which sounded pretty silly to me from the get-go).
If you're still having trouble seeing her spin in both directions, check out these images.