PyGTK is the Python binding for the GTK+ widget library.
PyGTK lets you easily create programs with a graphical user interface using the Python programming language and the GTK+ library. GTK+ provides all kinds of visual elements and utilities, and is the official toolkit of the GNOME Desktop.
PyGTK applications are multiplatform and able to run, unmodified, on Linux, Windows, MacOS X and other platforms.
For more information, see the Wikipedia page of PyGTK.