Here is an old one, but a good one and you can’t beat the price…FREE! XML Notepad is exactly what you would think it is, a stand alone XML editor that goes one step farther than Visual Studio by providing a tree view and an XSL output window for XML documents that have an associated XSLT transformation file. This is a handy tool that every programmer should have in his utility belt. The editor can be found at the Microsoft Download Center.
8/14/2021 Update: XML Notepad 2007 is no longer available through the Microsoft Download Center. Instead you can now find it on GitHub (https://github.com/microsoft/XmlNotepad).