Microsoft released Anti-Cross Site Scripting (XSS) Library V1.0 for .NET framework [
Details]
I think after SQL Injection, XSS is the second vulnerability which many web sites are infected to and so this tool can help (and awake!) many developers control this danger in their codes.
As said on download page of this library, current downloadable package only contains .NET 2.0 binaries and 1.x versions are due to release in future (Bad news :(( I still have to use 1.x version in some projects).
More information about XSS can be found
here.
UPDATE: Library updated! Now it supports both .NET 2.0 and 1.x.