Dr. Luis van Ahn presented in his fantastic Google Tech Talk Video called Human Computation (he speaks very fast sometimes in this video).
Luis van Ahn is one of the persons behind the CAPTCHA project and concept, if you haven’t already, check out the new reCAPTCHA project - it has some cool stuff like libraries and plugins for several scripts, Mailhide implementation, APIs (for PHP, Python, etc) and sounds - with reCAPTCHA you can provide real strong CAPTCHA security (unlike prior weak versions that are easily breakable) for your website while helping Internet Archive digitize books via OCR.

Another very nice CAPTCHA implementation can be found at http://www.netzgesta.de/S5/contact.php