If you are using CAPTCHA on your websites, watch this video. It’s an eye opener.
Now if you are wondering what are the alternatives, I suggest you to read this W3C Working Group Note: Inaccessibility of CAPTCHA – Alternatives to Visual Turing Tests on the Web . This note is pretty detailed and will guide you what is the best alternative.
I personally find asking logical questions (e.g. Is fire hot or cold?) pretty helpful. Make sure that you add a checkbox with the question that requires people to check or uncheck the question before they submit a form. This combination has worked pretty well in my personal projects.