I love illusions, as regular readers know. They play into astronomy as well into skepticism; you have to understand how your brain interprets images, or at least how it can be fooled, to correctly interpret astronomical imagery and antiscience claims (like pareidolia or cities on Mars).
I checked my blog archives, and incredibly I have never linked to Skytopia, a phenomenal collection of illusions. Well, I might have linked to it but I couldn't find it, so it's new to me, and maybe it is to you too. There are color illusions, geometric illusions, animated illusions, autokinetic illusions... it goes on and on. Go there, poke around, have fun, and exercise your optical nerve. How many examples are there that can be used to debunk antiscience claims?