Captcha

http://n3on.org/projects/reCAPTCHA/

http://n3on.org/projects/reCAPTCHA/ A paper [presented @ DEFCON 2010] describing algorithm to solve reCAPTCHA ( http://reCAPTCHA .net )

[rus] Взлом каптчи файлообменника / Искусственный интеллект / Хабрахабр

http://habrahabr.ru/blogs/artificial_intelligence/67194/#habracut how to break captcha using python and fann (FAst Neural Network )

CAPTCHA Service - Stop Guestbook Spam, Blog spam, Wiki Spam, Comment Spam - Protect Web Form .COM

http://www.protectwebform.com/ need a quick and dirty captcha? this is the place!

What is reCAPTCHA?

http://recaptcha.net/learnmore.html cool CAPTCHA that is used to OCR books

PWNtcha - captcha decoder

http://sam.zoy.org/pwntcha/ well, captcha decoder and the list of good and bad capchas