I was just helping a buddy of mine do some cross-site scripting (XSS) prevention and came across some good resources. If you don't know what XSS is, there is decent introduction here: http://www.counterhack.net/xss.ppt.
Now on to the ones that I really wanted to post:
This one is from Microsoft Patterns & Best Practices. How To: Protect From Injection Attacks in ASP.NET
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpag2/html/PAGHT000003.asp?_r=1
Here is a download for Microsoft Anti-Cross Site Scripting Library V1.0.
http://www.microsoft.com/downloads/details.aspx?familyid=9a2b9c92-7ad9-496c-9a89-af08de2e5982&displaylang=en
And this is a nice walkthrough of how to do code review for XSS vulnerabilities.
http://channel9.msdn.com/wiki/default.aspx/Channel9.CodeReviewXSS
Oh, and I almost forgot! This one gives you a load of examples to check your site for XSS.
http://ha.ckers.org/xss.html