Well, these are for linux/unix, but they may exist for windows, too:

$ debtags search 'devel::ui-builder && uitoolkit::gtk'
gazpacho - GTK+ User Interface Designer
glade - GTK+ 2 User Interface Builder
glade-gnome - GTK+ 2 User Interface Builder (with GNOME 2 support)
gtk2.0-examples - Examples files for the GTK+ 2.0
libgai-dev - Easy to use interface to create applets & dockapps
libgai0 - Easy to use interface to create applets & dockapps
libglade2-ruby1.8 - Libglade 2 bindings for the Ruby language
monodevelop - Development Environment for GNOME
pida - Python Integrated Development Application, a Python IDE
thewidgetfactory - a showcase for GTK+ widgets
vdk-doc - The Visual Development Kit C++ library
vdk2-tutorial - Tutorial for the Visual Development Kit C++ library 2
[download]

Oh, and I forgot to mention WxWidgets (WxPerl) in the list of alternative toolkits. Many people seem to like that one.