GTK+ (GIMP Toolkit) - cross-platform widget toolkit for creating graphical user interfaces

GTK+ is free software and part of the GNU Project. However, the licensing terms for GTK+, the GNU LGPL [http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html], allow it to be used by all developers, including those developing proprietary software, without any license fees or royalties.