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: http://www.python.org/doc/current/lib/module-Tix.html
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 [http://web.archive.org/web/20050307041957/starship.python.net/crew/mike/Idle/ archived] with sources [http://www.sourcefiles.org/Programming/Development/Environments/idestudio-1.9-linux-x86-350.tar.gz here]).