Well XML Spy is the daddy of XML editors... at a cost! http://www.altova.com/products/xmlspy/xml_editor.html I wish I could convince them to buy it here, only got an old version of the free "home" edition which they don't do any more, the grid view is fantastic when trying to work through XML
I highly recommend oxygenxml. I bought a personal license for learning about HTML and it was only $25 or so... It's very powerful and has a WYSIWYG view.
Try Spket. It's not for XML only. It's the best editor for JavaScript i've ever seen. Some benefits: * it's free * standalone * very fast * it has codeassist * visual tree-model of your code
If you're use Eclipse, you also can download Spket as Eclipse plugin.
Hope its okay to update this thread, I have been using Liquid XML Editor which also has a wysiwyg interface, gain its not free but thats why it has more features han the freeware kind.