wxPython
a blending of the
wxWidgets C++ class library with the Python programming language
A wxPython GUI can be built in any IDE. All you have to do is download and install the wxPython Package
for your version of Python. I used Notepad++ and Python 2.6.
Typically a wxPython GUI is made of 2 nested classes in a module. The first of which defines the frame:

Here is the weird, but cool, binding that wx does:

Here you can see some of the event handlers being implemented:

The second of of the two classes is the actual app class itself:

At the end of the module the app is ran:

Here's what the GUI looks like after all the code has been written:

I won't go into all of
what it takes to create a GUI in wxPython, but suffice it to say it is
much more difficult than any other GUI builder you have probably ever
used. More complex than Java and WAY more complex than C# or any other
.NET language. If you're writing a program in Python and you need
a GUI my advice is to use Iron Python.
Links & Downloads
wxPython
Download wxPython
wxPython Tutorial
Another (and better) wxPython Tutorial
Click here to download my wxPython Calculator GUI Example Code
Feel free to use this code to experiment with wxPython