Very useful: PyDev on Eclipse
As much as I like Java for medium to large scale server side projects, I am getting more and more into Python. I am thinking of changing my title from "Java consultant" to "Java and Python consultant" :-)
Python as a language and a platform hits the 'sweet spot' for rapid development of small and medium scale projects.
Another great feature that I like about Eclipse is that it is as a platform for writing and running your own tools (the book "Contributing to Eclipse" is a good reference for this). I tend to enjoy experimenting with many tools and technologies, but if I start doing more work in Python (I am mainly just working on a GPLed fun project in Python right now), I might switch back to Eclipse as my main development environment.