Depending on what platform(s) you are aiming at, there are several.
Standard builds of Python include an object-oriented interface to the Tcl/Tk widget set, called Tkinter. This is probably the easiest to install and use. For more info about Tk, including pointers to the source, see the Tcl/Tk home page at http://www.tcl.tk. Tcl/Tk is fully portable to the MacOS, Windows, and Unix platforms.
wxWidgets is a portable GUI class library written in C++ that's an interface to various platform-specific libraries. wxPython is a Python interface to wxWidgets. wxWidets supports Windows and MacOS; on Unix variants, it supports both GTk+ and Motif toolkits. wxWidgets preserves the look and feel of the underlying graphics toolkit, and there is quite a rich widget set and collection of GDI classes. See the wxWidgets page for more details.
There are bindings available for the Qt toolkit (PyQt) and for KDE (PyKDE). If you're writing open source software, you don't need to pay for PyQt, but if you want to write proprietary applications, you must buy a PyQt license from Riverbank Computing and a Qt license from Trolltech.
PyGTk bindings for the GTk+ toolkit have been implemented by James Henstridge; see http://www.pygtk.org.
Python bindings for the FLTK toolkit, a simple yet powerful and mature cross-platform windowing system, are available from the PyFLTK project.
A wrapper for the FOX toolkit called FXpy is available. FOX supports both Unix variants and Windows.
For OpenGL bindings, see PyOpenGL.
CATEGORY: gui