The mob at http://recaptcha.net/ got a bright idea. Take the 60 million CAPTCHA’s being solved each day and make it a helpfull task. digitizeing books!
They have taken a process of authenticating a human wanting access to a web site and added the ability to help digitize books. Help the OCR software that is. They take the words that the OCR software can’t figure out and place it as a CAPTCHA picture and have the people give them the answer.
The service is available for free as far as I can tell and also has a nifty URL builder called mailhide, Which I use. Mailhide protects your email address on the web from being picked up by spammers and helps to digitze books too. Pretty nifty.
Check it out at http://recaptcha.net/