Submitted by Your Computer Genius on Mon, 2007/12/03 – 9:00pm.
reCAPTCHA is not your normal “CAPTCHA” program.
CAPTCHA is a general name for the anti-spam, anti-abuse code that people place on their websites, especially on comments sections for blogs, or new account registrations, to prevent bots or automated computer programs from exploit or over-using web services.
As they state on their website, 150,000 hours of human time are spent solving CAPTCHA-style problems, or filling out CAPTCHA-style forms.
reCAPTCHA captures and channels that energy, adapting it to a useful and usable form — namely it aids in transcribing text from digital images into text, forming a critical link in the digitizing of physical books and the archiving of human knowledge.
The reCAPTCHA system uses this available source of human work to help with this archival process when automated OCR runs into problems.
You can tap into this system for free, and use it on your website to filter out spam and non-human users, all the while aiding this noble project.