Captchas are widely used in the Internet to differentiate between humans and computer. The basic idea is to detect automated programs by using a simple challenge that supposedly any human would be able to respond and subsequentially deny service.
Recent research on this area is focusing on exploiting these “human cycles” for meaningful human computation. For example, Luis Von Ahn from CMU has developed games that, when played by humans, label images and objects inside (ESP and Peekaboom).
Recaptcha is another interesting project whose purpose is to help the process of digitizing old books. Every scanned image of a word not recognizable by OCR is used as a Captcha. To confirm the correctness of the human input every time a scanned image of a word is fed into the system it is combined with a known word. If the answer for the known word is correct then a correct answer is assumed for the unknown word too. The main website contains more detailed information. I downloaded their plugin for Wordpress and I’m currently using Recaptcha to avoid receiving spam comments. I may have very few posts from third party but I receive hundreds of spam comments every month.