I love optical illusions (see Related Posts at the bottom of this article for more), so I was happy to discover that every year, the Neural Correlate Society has a contest to find the best ones. This year's Top 10 finalists have been announced.
My favorite (which I can't embed here, sorry) is called "Grouping by contrast". You'd swear the spots are not the same color, but I cut two holes out of a piece of paper to block the background, and sure enough they are. It's incredible. The illusion is a variation on the well-known chessboard contrast illusion and is similar to another illusion showing that how we see colors isn't as straight-forward as you might think.
Another cool one is this:
Click play, and stare at the white dot in the center. You can easily see the dots changing color... until the dots appear to rotate. They suddenly seem to stop changing color, but in reality they still are. Rerun the video and look at the dots instead of the center, and you'll see.
The best illusions are the ones where you'd swear it's a trick. But that's the whole point, isn't it? So go to the list and check out the other finalists. And remember: seeing is not believing. You cannot always trust your eyes and brain to give you a good picture of reality.
As a skeptic, I find that knowledge comes in handy quite often.
Tip o' the Escher staircase to slashdot.