wxWindows is a free cross-platform
GUI toolkit for C++.
It is available for Windows and several Unix
platforms that support GTK+ or Motif.
A stable Macintosh version should be available soon.
Language bindings for Python (wxPython) exist and
Perl-support (wxPerl) is under development.
Native widgets are used on every platform, so it's a
a good alternative to GTK+ Win32 if real
windows-look and feel is desired.
But many several small features are platform-specific,
so it's not the legendary Write Once, Run Anywhere tool.