CAPTCHA server control used by Търговски регистър : http://www.codeproject.com/KB/custom-controls/CaptchaControl.aspx?display=Print

Tools .NET info from web pages and server (get state, etc.):http://www.pluralsight.com/tools.aspx

http://www.webappsec.org/lists/websecurity/archive/2005-05/msg00088.html
http://www.codeproject.com/KB/custom-controls/CaptchaControl.aspx?fid=124885&fr=1#xx0xx
http://ww.megite.com/discover/captcha+security+programming+php
http://www.codinghorror.com/blog/archives/001067.html