The Tix (Tk Interface Extension) module provides an additional rich set of widgets to TkInter. Although the standard Tk library has many useful widgets, they are far from complete. The Tix library provides most of the commonly needed widgets that are missing from standard Tk.
Lib/lib-tk/Tix.py is a standard library module; the source distribution of
Python has a set of demos in Demo/tix/ and it is described in the reference manual:
To take a look at an example of how Tix can improve the look of a TkInter application, look at the 'Tixed' version of IDLE called IDEStudio at http://starship.python.net/crew/mike/Idle/ (not accessible anymore but archived with sources here).