From what I can see, the two main contenders are tkinter (ships with the default distribution) and wxPython. The former is easier to learn, has a more “consistent” look, (which basically means it looks terrible everywhere?) and is more suited for quick-and-dirty tasks, etc…
WxPython seems to be the future, has more features, is harder to learn, is more typical of GUItookits, is more actively developed, trickier to install (for users), …
There is a chooser here.
Update[2007-11-10]: What was was I thinking when I wrote this post? There’s pyGTK and pyQT; why not use one of them? I don’t even know now. Anyway, an old article about toolkits.
[2007-11-20]: Also noticed I had another (private) post that was a link to:
learning python » Blog Archive » Creating a GUI using Python, WxWidgets, and wxPython
Random intro-to-python link I’m putting here: Python in a few minutes