http://en.wikipedia.org/wiki/CAPTCHA

http://en.wikipedia.org/wiki/CAPTCHA

http://en.wikipedia.org/wiki/CAPTCHA

http://bs.wikipedia.org/wiki/CAPTCHA

http://www.captcha.net/

http://www.cs.sfu.ca/~mori/research/gimpy/

http://www.google.com/recaptcha/learnmore

http://www.slideshare.net/valicac/captcha

http://os2.zemris.fer.hr/ostalo/2009_zuzak/CAPTCHA.html

http://hr.mojoportal.com/konfiguracija-captcha-za-prevenciju-spama.aspx

http://www.domagoj.pavlesic.com/post/Captcha-i-digitaliziranje-starih-knjiga.aspx

http://security.lss.hr/documents/LinkedDocuments/NCERT-PUBDOC-2010-06-302.pdf

http://www.phpcaptcha.org/captcha-gallery/

http://spamfizzle.com/CAPTCHA.aspx

http://www.bir.ba/index.php?Itemid=84&option=com_zoo&view=category&category_id=2