EasyGUI
Need a really simple GUI really quickly? Get EasyGUI from http://www.ferg.org/easygui/index.html.
References
Quick guide http://www.astro.washington.edu/owen/TkinterSummary.html
Handy hints http://www.astro.washington.edu/owen/ROTKFolklore.html
"Thinking in TkInter" http://www.ferg.org/thinking_in_tkinter/index.html
Reference manual (PDF) http://infohost.nmt.edu/tcc/help/pubs/tkinter.pdf
From the Python docs http://docs.python.org/lib/module-Tkinter.html
also
The manual for Tcl/Tk -- the reference for really detailed stuff http://www.tcl.tk/man/