GTK+ is a multi-platform toolkit for creating
graphical user interfaces (GUIs). 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.
[Source GTK+ Web site]