FAQ: Is my Drupal site's contact form vulnerable to spam bots?
Portland State uses a free CAPTCHA program, called reCAPTCHA, to tell whether a user is a human or a computer. As part of completing the contact form, users view distorted text (or listen to spoken words) and then type those words into the space provided. Since computer programs cannot read distorted text as well as humans can, a CAPTCHA program provides excellent protection against spam bots.