The McCollough Effect. Here's another good, if shorter-lived visual effect. And, if you're feeling like crossing sensory modalities (and why wouldn't you be!?), I provided some Shepard Tone links in this long, long post about audio compression.

Man. It seems so long ago that I could waste time writing thousands of words every day about nerdy stuff. Good times.

But back to my point: brains are weird, and terrifying. I love these sorts of demonstrations, where our inherent limitations are laid bare. If there's one thing humans are bad at, it's noticing our own shortcomings. And I don't just mean that in a dippy, navel-gazing, self-improving, let's-all-hold-hands-and-sing sort of way. I mean it in the you can go blind and not realize it sort of way. Hell, we've all got a gap in our visual fields where the optic nerve exits the retina — we just don't realize it (figuring out if/how the brain "papers over" this blind spot was a particularly tedious and thoroughly-investigated area of neuroscience, if I remember correctly).

Anyway, I've always thought that our inability to naturally recognize these sorts of limitations is a good thing to keep in mind — to whatever extent our puny human brains are capable of genuinely believing in their puniness, that is.