A coworker sent me a link to Microsoft's XML Notepad 2006 today. According to the download page, "XML Notepad 2006 provides a simple intuitive user interface for browsing and editing XML documents."
I downloaded it just to give it a quick try. It opens XML files in a tree view, similar to Eclipse's default XML editor. It also looks like it can do XSL transformations, but I couldn't get this to work. (To be fair I really didn't try that hard, I just wanted to see what the tool looked like.)
Anyway, some people may find this tool useful, but as I'm an Eclipse user I think I'll stick with XMLBuddy for now.