I am a big fan of optical illusions; I love 'em (see here and here and here and here). A lot of Bad Astronomy is due to one illusion or another, like the Glass Worm, but that's incidental. I just think they're cool.
One of my favorites is the in-out illusion, where your brain can't tell if something is convex or concave. It's maddening, and fun. The Mighty Optical Illusions site has one of the best examples I have ever seen of it, a spinning silhouette that'll melt your brain. Go have a look!