GTK+ 3 also known as Gtk3 is a multi-platform GUI toolkit, it is written in C but has bindings for a lot of languages including C++, Python, Vala and Ruby. (For the full list see the Gtk website).

Gtk+ is part of the GNU Project and falls under the GNU LGPL licences meaning it is allowed to be used by all developers, including those developing proprietary software, without any license fees or royalties.

Credits: loosely based on http://www.gtk.org/