The Illusion Contest of the Year recently announced their top ten finalists, and the overwhelming crowd and jury favorite is Impossible Motion: Magnet-like Slopes by Koukichi Sugihara of the Meiji Institute for Advanced Study of Mathematical Sciences in Japan.
Ready? Brain-trip on this:
"In this video, wooden balls roll up the slopes just as if they are pulled by a magnet. The behavior of the balls seems impossible, because it is against the gravity. The video is not a computer graphic, but a real scene. What is actually happening is that the orientations of the slopes are perceived oppositely, and hence the descending motion is misinterpreted as ascending motion. This illusion is remarkable in that it is generated by a three-dimensional solid object and physical motion, instead of a two-dimensional picture."
Not bad, Koukichi. Physically constructed object. No computer tricks used. (Not to mention, over three million views and counting on YouTube in a month's time.)
Check out the rest of the finalists here.