Unfortunately CAT.NET is as good as dead so you can rule that one out.
In terms of XSS detection, if you're wanting to test vulnerabilities in an existing app you need not constrain yourself to static code analysis or .NET specific tools. There are plenty of language agnostic scanners out there you can run against the URL of an existing site.
One I've found very useful in the past is Netsparker from Mavituna Security. This is really easy to get up and running with (download it, enter a URL in a nice UI, run the scan) and the results are easily interpretable. You can grab a free community version which will identify persistent and reflective XSS (among other vulnerabilities) or pay for a licensed edition with a heap of additional features.