I’ve always loved optical and other illusions, as the prove that even the software that runs inside our very own skull isn’t error prone either, but fortunately a lot more stable then your average Windows operating system…
A site that seems to be covering a lot of them is Grand Illusions, and they even have some really neat WMV videos up for those visual effects you really have to see with your own eyes to believe.
The Dragon Illusion is excellent for instance, and there instructions on how to achieve the effect yourself on the site as well.
Neato!! Now where did I put my scissors again…