It's no wonder that Google decided to acquire reCAPTCHA and use the service to improve Google Book Search's digitizing accuracy.
"reCAPTCHA's unique technology improves the process that converts scanned images into plain text, known as Optical Character Recognition (OCR). This technology also powers large scale text scanning projects like Google Books and Google News Archive Search. Having the text version of documents is important because plain text can be searched, easily rendered on mobile devices and displayed to visually impaired users. So we'll be applying the technology within Google not only to increase fraud and spam protection for Google products but also to improve our books and newspaper scanning process."
The service offers a simple JavaScript API that allows you to embed CAPTCHAs in any web page and many popular sites use it: Facebook, Twitter, Ticketmaster.