CAPTCHAS invoke a number of reactions amongst both designers and users. They are a common source of frustration, consuming screen real estate and more importantly the user’s time spent deciphering and completing them. Studies have shown that they are generally bad for business.

However they are still widely used as they are considered a necessary evil in protecting websites against bots by generating tests that humans can pass but current computer programs cannot.

As you may have heard Google recently purchased ReCAPTCHA to accelerate its massive efforts to scan tens of millions of books and periodicals.

What you may not know is just how Google is utilising this technology to actually use the (re)CATPCHAS you’re filling out. By completing a reCAPTCHA you are directly helping Google in their quest to digitise the world’s books.

Here is a surprisingly interesting presentation on what is a generally dull topic: Why Nerds Rule: Luis Von Ahn and reCAPTCHA

UPDATE: CAPTCHA’s: Tough on Sales & Common Way to Test User Tolerance | UX Booth