http://cintruder.sourceforge.net/

• pentest
• tools
• hacking
• cracking
• captcha
• security

http://www.webdesignfromscratch.com/javascript/human-form-validation-check-trick/

• forms
• validation
• captcha
• spam

http://www.phpcaptcha.org/

• captcha
• php
• spam

http://www.ocr-research.org.ua/list.html

• captcha
• security
• ocr

http://sam.zoy.org/pwntcha/

• captcha
• decoder

http://www.kgreene.com/captcha.php

• php
• captcha