In order to deal with a bunch of comment spam (which come in droves, always repeating variations on the words “viagra” and “slut”), I’ve found reCaptcha and have put its WordPress plugin on my blog. I think it’s an ingenious service: it forces you to write the correct spelling for two words. One is a word that the OCR readers working on digitizing old books can’t recognize; the other is a word that many reCaptcha users before you have consistently read a certain way and have thus provided the correct spelling for. So, if you get the second right, you’re likely to get the first right, too, because, well, you’re a literate human.
From poking around the net, though, I found the worst captcha solution: something I’m calling catCaptcha, for the lack of a better word. Take a look (hat tip: Depressed Programmer). The Rapidshare folks must have said, “Let’s make a captcha so hard that even humans fail at it most of the time.”