MyCaptcha is a free opensoruce ASP.NET Captcha control written in C# 3.0.
As described in Wikipedia, "A CAPTCHA or Captcha (IPA: /ˈkæptʃə/) is a type of challenge-response test used in computing to ensure that the response is not generated by a computer". In a crude term, a captcha control shows some hard-to-read letters on the screen and asks the user to enter the text in a box. Then checks to see whether the entered text is correct or not.
This project includes a free and open-source C# 3.0 (ASP.NET) control with the following features:
1. Is developed as an .ascx control so it is reusable in every ASP.NET website.
2. Saves nothing on your hard-disk so if you open a page in two different windows (or tabs) there will be no conflict between two Captcha controls.
3. Is very easy to use and needs no complicated concept like understanding and using http handlers.
4. One can use several instances on a page.
Please visit the author's page here: http://aspguy.wordpress.com