Microsoft has made available the XPS Essentials Pack that enables users to print out to the XPS document format and view XPS documents with a stand-alone XPS Viewer. XPS is natively supported already in Windows Vista due to the fact it ships with the .NET Framework 3.0 (which is available as a separate download for Windows XP too). However, I've been asked about a stand-alone XPS viewer application. Currently in Windows Vista, XPS documents are viewed within Internet Explorer 7. By installing the XPS Essentials Pack, you get the XPS Viewer EP which enables you to view XPS documents outside of Internet Explorer 7.
Download: XPS Essentials Pack 1.0
The default behavior in Windows Vista will still remain to have XPS documents opened in Internet Explorer 7 even with the XPS Essentials Pack installed.
You can change the default behavior by right-clicking on a XPS document, going to Open With and Choose Default Program. An option panel will appear that will let you change the default from "XPS Viewer" to "XPS Viewer EP". Make sure the check box for "Always use the selected program to open this kind of file" is checked.
If you are one of those people who would prefer to view XPS documents outside of Internet Explorer 7, this tool is just for you.
Thanks to Ben Kuhn for pointing this out.