Michael Bach collected 61 Optical Illusions and Visual Phenomena and provided explanations for most of them. There are many amazing illusions, but I especially liked the one that is shown on the right side of this post.
"Optical illusion" sounds pejorative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as bringing out particular good adaptations of our visual system to standard viewing situations. These adaptations are »hard-wired« into our brains, and thus under some artificial manipulations can cause inappropriate interpretations of the visual scene.