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.