I like this class library because it looks for "good things" and not "bad things."
The most common method of mitigating XSS issues is to use functions like HtmlEncode that look for "bad things" and escape them. But this library does the right thing - it looks only for good things (a very small set), and escapes everything else.
There are only two methods:
AntiXSSLibrary.HtlmEncode and AntiXSSLibrary.UrlEncode
Very cool... and very simple.