There are a lot of sites including Google that use a CAPTCHA to distinguish a human visitor from an automatic web robot (called bot). An image is shown containing some letters and the visitor is required to recognise the letters. This is something that’s difficult for computers to do, but easy for humans.
reCAPTCHA is an interesting, free for all CAPTCHA project but it has a dual use: on the one hand (like all CAPTCHAs) it helps prevent spam, and on the other hand, it helps digitize books. The words it shows are taken from books that have been scanned but the OCR software is finding difficult to understand. More information on the reCAPTCHA website. I am planning to use this soon on my crossword solver page. I recommend that people add this to their websites – its very easy to do so with the plugins that are available.
By the way, CAPTCHA stands for ‘Completely Automated Public Turing test to tell Computers and Humans Apart’