Preventing Cross Site Scripting (XSS) Attacks
If you are developing real world ASP.NET applications then security is something that you should be concerned about. How do you make your site or application is tamper proof but still make it flexible enough from an end user point of view ?
In order to make your web application more secure then you should take a look at the Microsoft Anti-Cross Site Scripting Library
Also, to learn more about preventing Cross Site Scripting in general there is a good MSDN Patterns and Practices article available at How To: Prevent Cross-Site Scripting in ASP.NET