GTK+: UI for your Linux Application
GTK+ is a toolkit written in C to make UI (User Interfaces) for your Application.
It is licensed under GNU LGPL 2.1, so it can be used by all developers even the software being developed is proprietary without license fees or royalties.
GTK+ is not only limited to Unix/Linux OS but it can be also used with Windows or MAC OS.
Just like other UI Toolkits (e.g. Swing for Java, MFC, Windows Forms), it offers the same functionalities like Window, Buttons, etc.
Here are sample screenshots of applications using GTK+:
Here are some useful links in developing application with GTK:
Beginner’s Tutorial: http://zetcode.com/tutorials/gtktutorial/
For more details about GTK+, head to its official webpage: http://www.gtk.org/