Google has re-introduced re-captcha, an alternative to the 2000 user dialogue challenge-response test used in computing and on the Web to determine whether or not the user is human.
http://www.google.com/recaptcha/intro
Instead of asking customers/users to re-enter 2 distorted words, which has been known to have accessibility issues, the new system uses IP address detection and cookies to reduce the user input to a simple checkbox.
Brilliant!
Update (3 Dec 2014): Wired has written a lengthy article on the new Google 1 click reCaptcha