You've probably seen website sign-up forms with a graphic of some
crazy-looking twisted alphanumeric characters that you're asked to type into a
form. That's to ensure that an actual human being has to do the signing up. It's
called text Captcha (Completely Automated Public Turing test to tell Computers
and Humans Apart).
A novel approach to this concept is to display a series of pictures and
instruct the website visitor to click on the kittens, or in the case of
Hotcaptcha, click on the three attractive young ladies (or men, your choice) in
the picture. Even though looks are subjective, machines can't tell the subtle
differences between a plain Jane and a hot babe, at least not yet. The code is
available to use on your own website. Looks like more fun than just typing in
text. Nice idea.
Product Page [Hotcaptcha, via Propellerhead]