GTK+

("The GIMP ToolKit", or incorrectly "Gnu ToolKit" or "Generic ToolKit") 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+ consists of the three parts; GLib, providing basic data structures, event handling, threads, etc., Pango, for layout and rendering of text, and ATK, providing interfaces for accessibility.