FWIW the editor in Chromium's and Firefox's developer tools are pretty good. I'd really like to see something like that as a standalone tool for both trees and XML and see how far it can be taken.
The inspector? I've seen and worked with similar tools for XML, too, though not recently. They're very good for small, constrained hand-editing, but I find them to scale extremely poorly.
For XML you may want to take a closer look at XML Marker 1.1 (it's old, freeware and Windows but still better than many other tools) and maybe Eclipse.
I think that sums it up. We really can't top crufty old Windows freeware?
There is a gaping hole in the market.
The last time I worked on complex XML I even used XML Marker with WINE... it works there. If you want the Mercedes of XML editors you should take a look at Oxygen XML editor (this is not free but runs on more platforms than Windows).
+1 for OxygenXML - it is very actively developed and has its own CSS engine that goes above and beyond what browsers can do.
So you can style your tree with CSS.