Applications that support multiple windows should support multihead
displaying, i.e., it should be possible to put some of the windows
on a separate screen. If you are blessed with two screens it is
a very nice productivity improvement to use that extra screen for
reference materials relating to what you work on. Applications
that support this are cool.

Gnumeric is cool, of course. Emacs is cool. Mozilla is not cool.

Unfortunately multihead support is a bit of a late add-on for GTK+.
The duct tape is visible here and there. For example, every time you
create a widget for the non-default screen, you in effect build it
for the default screen with all the setting that relate to that,
then warp it over to the other screen and redo all the settings.
Anything that relies on settings — icon sizes, fonts, etc. — needs
to be done or redone when the widget is realized.

But GTK+ is better than the rest of the Gnome stack. There are
still places that build X cursors for the wrong display, fail to
catch screen-changed signals, or grab the cursor on the wrong
display. The results range from the cute (popups and menus show on
the wrong screen) over the irritating (application crash) to the
worse (grabbed X cursor).