For a CAPTCHA system, I heartily recommend reCAPTCHA.
Traditional computer-generated CAPTCHAs will eventually be broken by developing a sufficiently intelligent system. For instance, here's someone who claims to break the Google CAPTCHA, formerly considered unbreakable, with a 30% hit rate. reCAPTCHA, by definition, shows you only images that cannot be recognized by optical character recognition.
And at the same time, your users' effort will be directed towards the common good - they help digitize books by recognizing words that cannot be recognized automatically.
See here for further explanation and to try it out.
