找到几个，大家可以去看看：

http://www.codeproject.com/aspnet/CaptchaImage.asp

http://www.codeproject.com/aspnet/CaptchaControl.asp#xx1001265xx

http://www.codeproject.com/aspnet/CaptchaNET_2.asp

CAPTCHA stands for "completely automated public Turing test to tell computers and humans apart." What it means is, a program that can tell humans from machines using some type of generated test. A test most people can easily pass but a computer program cannot.

当然，最权威的在msdn上。虽然有些复杂，呵呵

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnaspp/html/hip_aspnet.asp

Human Interactive Proofs