I found a great free tool at Microsoft for validating XML and viewing XSL output of an XML file. I was working on the RSS feeds at Community MX and had a bit of problem with the OPML feed (my XSLT skills are rudimentary at best.) The tools install into Internet Explorer and add two new menu items to the right-click menu:

• Validate XML
• View XSL Output

This allows you to see the HTML code generated by the XSLT code. It's handier than some other tools because it's right in the browser.