tedd's CAPTCHA's
Refresh for new values
However:
CAPTCHA's are not 100% successful. They're designed to tell you if someone is 'human' or not, but not whether something is spam or not. Just because something came from a human being doesn't mean it isn't spam. So, don't depend upon them for your projects.
CAPTCHA's present accessibility problems for people with disabilities.
CAPTCHA's show the world that you care about the quality of the content on your site without requiring identification measures. But remember that we developers haven't given up trying to find a better way.