In various websites, Captcha control is used for validating the user input and protect web application from the program. Here is one of the sample control for Captcha. It is basic one which can be enhanced further by including more to the image generation and even including the timeout. Or you can add much more to it
This application has two parts
2. CaptchaControl Sample Project
Captcha control has three files.
1. CaptchaControl (Which is the main control)
2. CaptchaImage (Which generates the Captcha image)
3. CaptchaImageHandler (which will provide the image to browser from the cache).
You can download it from (Captcha Control Sample)
Captcha Sample Control