GTK+ is a somewhat cross-platform GUI toolkit that arose out of the GIMP (GNU Image Manipulation Program) project. It's primarily Linux based, but there is a Win32 port, which the GTK+ project seems to not want to take full responsibility for.
GTK+
From the GTK+ website :
GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off projects to complete application suites.
GTK+ is free software and part of the GNU Project. However, the licensing terms for GTK+, the GNU LGPL, allow it to be used by all developers, including those developing proprietary software, without any license fees or royalties.
GTK+ has been designed from the ground up to support a range of languages, not only C/C++. Using GTK+ from languages such as Perl and Python (especially in combination with the Glade GUI builder) provides an effective method of rapid application development.