Part of the standard Python distribution. Documented in Grayson and Lundh books [bibliographic info needed here].
Implemented through Tcl scripting interface, not the C API. Consequence: slow, but faithful.
Gustavo Cordero has worked on a recipe to make Tk extensions--tkhtml and tktable, most immediately--available to Tkinter developers.
The Python Wiki has its own page for Tkinter [L1 ].