Someone tried this http://www.xmlcooktop.com/? I was looking at https://www.hunterstone.com/hsstore/ProductDetails.aspx?productID=112 (Hunterstone Web.Config Editor). For sometime it was free but now it is a shareware.
XmlCooktop seems to be a promising free XML editor.
Instead of a dedicated XML editor, you could also try general purpose IDEs like Editplus (www.editplus.com) or Netbeans (though Netbeans is commonly used for Java coding it can also prove to be a good XML editor).