I use
Visual Web Developer Express 2008 from Microsoft. Very nice editor, you can apply css, transforms, and schemas to the xml file. The extensive options for customizing the editor behavior is also real nice, all of the Visual Studio Express allow you to create template files, like the template I use is just the barebones EQUI Xml file, so when I hit New my EQUI Xml Template shows up in the list. I think you would have a hard time finding a better editor once you get used to the VS08 stuff.