On the FuzzyOpinions blog, there's a basic tutorial that walks you through the creation of a CAPTCHA system to use how you'd like on your site. They target it towards the most common use - blocking unwanted comments to a site from spambots.

Although you might not know it by the name, a captcha is one of those little boxes you see, more and more lately, that ask you to type in a random code to verify that you are a human being and not a robot. There are many different varieties, but the basic idea is an image that is obscured slightly but readable by a human and used as a passcode for secure entry to a system or task.

The method is a simple combination of random text, a custom image, some of the PHP graphics functions, and a little addition to your HTML form (and PHP session) to get things up and running.