reCAPTCHA is one of the coolest things that I didn’t know about until today (I’m sure there is more).
They provide a free captcha service to any website that needs it. What does reCAPTCHA get in return? Each captcha that is entered by your users helps digitize pre-digital-age books. So when OCR fails, the word is farmed out as a captcha to participating sites!
Win-win = highest form of awesome :)