I'm actually surprised that i didn't see it before, Captcha (those squiggly words you have to type in to register or verify on a site) are actually used to write digital books!
if you don't want to read it all scroll-down to 'DIGITISING BOOKS'
Answer by kevin · Mar 03, 2011 at 08:17 PM
Standard captcha are usually used to prevent spammers or botters from creating things that will automatically create bulk accounts/registration/etc. Other forms like recaptcha uses the same basis, but also include another captcha to help identify words for Google servers (who own recaptcha).