I can't seem to find one or my google-fu fails me. Is there one?
A free converter web service is available from XPS2PDF. All you have to do is upload your XPS file, and it will give your a PDF version.
NiXPS Viewer 2.0.1 - This is the only one I can find and it's definitely not free.
It's free to try but costs 75 euros ($99 USD). The editor is 225 euros ($299 USD).
I use XPSReader from Mac Appstore... it cost only 15 and does everything I need. Here is the link to AppStore