Zend_Service_ReCaptcha provides a client for the reCAPTCHA Web Service.
Per the reCAPTCHA site, "reCAPTCHA is a free CAPTCHA service that
helps to digitize books." Each reCAPTCHA requires the user to input
two words, the first of which is the actual CAPTCHA, and the second
of which is a word from some scanned text that Optical Character
Recognition (OCR) software has been unable to identify.
The assumption is that if a user correctly provides the first
word, the second is likely correctly entered as well, and can be
used to improve OCR software for digitizing books.
In order to use the reCAPTCHA service, you will need to sign up for an account and register one or more domains with the service in order to generate public and private keys.