Google has launched reCAPTCHA in order to help digitize books. Oh, yeah, this came as a surprise to you as well? It did for me at least. Actually, this is one of the reasons I love Google. They computed that we write about 200 million CAPTCHAs on the web every day. Each CAPTCHA takes about 10 seconds to write and thus we waste about 500.000 hours every day writing CAPTCHAs.
To make a long story short. Google decided that they wanted to do something with this otherwise wasteful use of time. As they are working on digitizing all the books in the world, they decided that they should put the two problems together and make them work for each other.