solving CAPTCHAs to transcribe books?

Old article, but still a good read!

In short, reCAPTCHA has found a nifty way to crowdsource book transcriptions. When users solve a CAPTCHA through reCAPTCHA, the software will give users two words: one with a known answer (the control word) and one where the OCR software wasn’t quite sure what the word was. Once a certain number of users have solved the suspicious word with the same result, it becomes a control word itself and the OCR software can learn this word.