DevShed.com: Asirra Captcha PHP Integration
Note: This article was originally published at PHPDeveloper on 30 July 2010.
Unlike other types of captcha that utilize difficult text obfuscation techniques (such as Google reCaptcha), this system utilizes images of dogs and cats, such as those shown in this screenshot.
They describe some of the reasons to use the system (hard to break, doesn't use sessions, easy to integrate) and how it works. They show how to implement the system on both the server and client side.