I thought this was a pretty cool implementation of CAPTCHA.
http://www.networkworld.com/newsletters/sec/2008/091508sec1.html?page=1
The CAPTCHA is the squiggly word that appears on Web sites to stop bots from sending spam and doing other vile deeds. Recently, several computer scientists reported on an innovative application of CAPTCHAs: potentially using the more than 100 million applications of human intelligence in decoding the symbols for useful work.
http://www.networkworld.com/newsletters/sec/2008/091508sec1.html?page=1