This site is pretty cool: purves-lab’s “See For Yourself” section. It lets you interact with various optical illusions, many famous, but quite a few I’d not seen presented before.
This illusion is very famous: which one of the two line segments is longer? You know the answer, of course; they are both the same length. But this page adds a little twist by letting you adjust the bottom line’s length until they look even to you, and reports the results. For me, there ended up being a 16% difference in perception versus reality.
Many of the illusions are more maddening (and fascinating) than this.