Here's a nice optical illusion. A bunch of almond-looking things arranged in a particular pattern appear to undulate. (Also, don't miss the link to the confounding checkershadow illusion, which I wrote about a couple of years ago in BB.) Link
Bill Beaty points out that the creator of this image ( Akiyoshi Kitaoka, who is uncredited in the site linked above) has a lot of other optical illusions, which are well worth checking out.