Look at this: http://glinden.blogspot.com/2009/05/exploiting-spammers-to-make-computers.html http://glinden.blogspot.com/2009/05/exploiting-spammers-to-m... "

Look at this: http://glinden.blogspot.com/2009/05/exploiting-spammers-to-make-computers.html http://glinden.blogspot.com/2009/05/exploiting-spammers-to-m... "Googlers Rich Gossweiler, Maryam Kamvar, and Shumeet Baluja had a fun paper at WWW 2009, "What's Up CAPTCHA? A CAPTCHA Based On Image Orientation" (PDF), that asks people to rotate images correctly to prove they are human rather than the norm of deciphering distorted text."