IDEs, Extensions, and Tools 

 IDLE (included in standard distribution)  free
 Pythonwin (included in win32all install)  free  
 Boa Constructor - wxPython GUI Builder  free
 Komodo v1.2  $295
 ActivePython v2.0 - MS Visual Studio plugin  $295
 PythonWorks Pro v1.3  $395

Useful extensions and tools

Numeric -- fast numerical computation (arrays, matrix manipulation, FFT, and more)
documentation (PDF)
Psyco -- specializing compiler akin to a JIT
Pyrex -- Python to native libraries
PyInline -- imbedded C code in Python scripts
pSQL -- makes SQL access look like dictionary access
Anygui -- a wrapper API for a large number of underlying graphic toolkits
SWIG -- "automatically" wraps C/C++ code to make Python extensions
Tkinter -- GUI toolkit included in the standard Python distribution and installation
Pmw -- Python MegaWidgets, general extensions to the Tkinter GUI toolkit
BLT -- MegaWidgets for graphs
Plotting -- various libraries
PIL -- image processing
mcrypt -- cryptographic routines
Zope -- a free, Open Source web application server