I maintain a Drupal module, Honeypot (<a href="http://drupal.org/project/honeypot" rel="nofollow">http://drupal.org/project/honeypot</a>), that does this and also adds in a few more protections, like a minimum time limit for submission (exponentially increasing, along with integration with Drupal's flood control mechanism.<p>I've found it to work extremely well on even some websites specifically targeted by human/automated spammers.