Most likely the best online source for optical illusions , with concise explanations and plenty of further references for the very interested. You should perhaps start by reading this very short pdf file , in order to get some idea of the scientific questions involved here. Some of my personal favourites (most of them "classics") among the examples discussed: "sigma motion", "biological motion", "lilac chaser", "Fraser's spiral", "Mueller-Lyer illusion", "hidden bird", "Dr. Angry and Mr. Smile" and, of course, "face in blocks", at its best in Salvador Dali's 'Gala-Lincoln' trompe l'oeil masterpiece.