reCAPTCHA is a CAPTCHA with a difference. Many books are being scanned digitally for posterity but, as well know, character recognition is not perfect – often words can’t be read by a computer. So when you use reCAPTCHA is presents you with two words. One is a word that couldn’t be read by OCR. The other is a word that is known. You don’t know which is which. During the CAPTCHA process you enter both words. If the word that it knows is correct is correct, then it assumes the other is too. When the unknown word is tried by a number of people, all giving the same answer, they know what the word is and can update their digitized document.
So, in a nutshell, it helps digitize books. And what a fantastic idea that is.
It’s also free and provides a speech method for those with sight problems.
It can be found on the Copy+ contact pages.