DrGaius:
17 Scintillating Grid

This is a variation of the Hermann Grid where black dots appear and disappear at the intersections of the gray lines. Interestingly enough, if you cock your head at a 45 degree angle the effect is reduced (but not eliminated).

DrGaius:
20
Monster Illusion
Found in virtually every psychology textbook in the world, the two monsters in this optical illusion are in fact the same size. Your brain automatically adjusts images that it perceives to be distant in order to compensate for the fact that they are larger than they seem.