http://www.captcha.net/
http://www.poslovni-informator.com/index.php?option=com_content&task=view&id=10896&Itemid=233
http://www.bug.hr/vijesti/naprednija-captcha/96353.aspx
http://www.codeplex.com/subkismet
http://hr.mojoportal.com/konfiguracija-captcha-za-prevenciju-spama.aspx
http://hr.wikipedia.org/wiki/Captcha