Please don't hesitate to expand this answer.
It's build on top of Qt, a C++ framework. It's quite advanced and has some good tools like the Qt Designer to design your applications. You should be aware though, that it doesn't feel like Python 100%, but close to it. The documentation is excellent
This framework is really good. It's being actively developed by Trolltech, who is owned by Nokia. The bindings for Python are developed by Riverbank.
PyQt is available under the GPL license or a commercial one. The price of a riverbank PyQt license is about 400 euro per developer.
Qt is not only a GUI-framework but has a lot of other classes too, one can create an application by just using Qt classes. (Like SQL, networking, scripting, …)
Qt used to emulate GUI elements on every platform but now uses native styles of the platforms (although not native GUI toolkits): see the documentation for Mac OS X and the windows XP style
Packaging is as simple as running py2exe or pyInstaller. The content of my PyQt app looks like this on windows (I have used InnoSetup on top of it for proper installation):
pyticroque.exe PyQt4.QtGui.pyd unicodedata.pyd
MSVCP71.dll PyQt4._qt.pyd unins000.dat
MSVCR71.dll python25.dll unins000.exe
PyQt4.QtCore.pyd sip.pyd _socket.pyd
QT comes with a widget designer and even in recent versions with an IDE to help design Qt software.
PySide is a LGPL binding to Qt. It's developed by nokia as a replacement for the GPL PyQt.
Although based on a different
technology than the existing
GPL-licensed PyQt bindings, PySide
will initially aim to be
API-compatible with them. In addition
to the PyQt-compatible API, a more
Pythonic API will be provided in the
future.
wxPython is a binding for Python using the wxWidgets-Framework. This framework is under the LGPL licence and is developed by the open source community.
What I'm really missing is a good tool to design the interface, they have about 3 but none of them is usable.
One thing I should mention is that I found a bug in the tab-view despite the fact that I didn't use anything advanced. (Only on Mac OS X) I think wxWidgets isn't as polished as Qt.
wxPython is really only about the GUI-classes, there isn't much else.
wxWidgets uses native GUI elements.
Others
I haven't got any experience with other GUI frameworks, maybe someone else has.