IDEs with code completion and other features.

Komodo - Multi-language IDE with support for Python 2.x and Python 3.x
PyCharm - Free open-source IDE with a smart Python editor providing quick code navigation, code completion, refactoring, unit testing and debugger.
WingIDE - Commercial Python IDE with advanced debugger, editor with vi, emacs, visual studio and other key bindings, auto-completion, auto-editing, snippets, goto-definition, find uses, refactoring, unit testing, source browser, and much more.
NetBeans - Python/Jython support in NetBeans. Open source, allows Python and Jython Editing, code-completion, debugger, refactoring, templates, syntax analysis, etc.
VS Tools for Python - Open-source plugin for Visual Studio 2010, 2012 and 2013. Supports syntax highlighting, debugging and rich intellisense, refactoring, object browser, MPI cluster debugging, Django intellisense and debugging, development REPL window and a debugging REPL window.
Atom - 4th Generation enhanced GUI, rich integrated enviornmet for multiple languages including python.

Debugger

pudb - A visual, console-based, full-screen debugger, designed as a more comfortable drop-in replacement for pdb.
pyDebug - Debugger for Python programs with a graphical user interface.
gdb - Windows Pyclewn allows using Vim as a front end to a debugger. Pyclewn currently supports gdb and pdb.

pyUnit - Unit testing library for python.
zopeTesting - Provides a powerful test runner that supports test discovery and a wide range of options to control how tests are run and results reported.