Glade http://glade.pn.org/ has already been mentioned, though that URL isn't
working for me. Parrot http://www.vision25.demon.co.uk/prog/parrot.html
doesn't count because it doesn't give you a GUI for designing and editing.
Please add any relevant open source tools and formats that we should be
aware of.

I would prefer something that doesn't generate code, but if code is created,
then the generated code should reference separate resources rather than
doing all the menu, dialog, window building inline.

The two things I know best are ResEdit from my Mac days and VB. I think
ResEdit (not regedit the registry editor on Windows) would probably be best.
VB ends up sticking the widget descriptions in .frm files along with the
code, but it does give you a GUI interface to doing layout and editing
widget properties without having to resort to code. There is a similar tool
for Visual C++ on Windows, but I don't know what it is called and what
format the files are saved in.

For wxPython, there is Boa Constructor
http://sourceforge.net/projects/boa-constructor/ but Boa generates code, not
resources. A simpler version of Boa could just generate resource .rc type
files or somebody's favorite XML resource format?

wxDesigner http://www.roebling.de/ is a commercial (the free version can't
do Saves) and much more elaborate GUI designer for wxWindows/wxPython/wxPerl
and apparently wxDesigner supports importing some form of XML resources. I
haven't used it.

Having a tool capable of building resource files for describing windows,
menus, widgets is a good first step towards a PythonCard.

ka