GTK+

GTK is a highly usable, feature rich toolkit for creating graphical user interfaces which boasts cross platform compatibility and an easy to use API.

GTK it is written in C, but has bindings to many other popular programming languages such as C , Python and C# among others. GTK is licensed under the GNU LGPL 2.1 allowing development of both free and proprietary software with GTK without any license fees or royalties.