I'm currently using IDLE, its decent, but I'd like to know if there're better lightweight IDEs built especially for Mac — free or commercial.
TextMate is a great general-purpose editor for mac. Used by the Rails community a lot, but it's great with python too.
Purpose built for Mac? Xcode supports python. Textmate and BBEdit are also great editors.
If you're fine with generic multiplatform IDE's, Komodo Edit is great.
There is a commercial one - Textmate. Most of the good free editors are cross-platform (if you are ok with it, I'd recommend EditRa - but it doesn't work properly under 10.6 yet, because of some bugs in wxPython).
There're aren't any Mac-specific Python IDEs, though Komodo's IDE, NetBeans and Eclipse (with the excellent PyDev extensions) all have very good Python IDE functionality and run fine on OS X. I've never used Wing IDE on OS X, but it has a pretty loyal following as well. If you just need an editor (I find that a good editor and a command line are often sufficient for Python), TextMate along with IPython in a terminal is the way to go. When I really do need a full IDE, I've been very happy with Eclipse/PyDev.
vim, emacs, BBEdit, WingIDE, or my favorite - eclipse (although, I don't think this is very lightweight)
I would recommend you to look at Aptana(it's more attractive then Eclipse for me)+PyDev or PyCarm. I use TextMate too, but those are easy for debug.
I use FRAISE (free) it is simple and useful, auto indentation, colorize, auto completion, shell.