I've been using the open source Eclipse IDE for a couple of months to write Java code. Eclipse is heavy, but it has three great features that motivate me to fire up the IDE rather than Xemacs:

• Code completion tied into import statements. Code completion alone isn't a big deal to me, but what's special in Eclipse is that code completion automatically generates import statements. If I want to use some class in my code, I type the class name then hit ctrl-space. A menu pops up asking me which class I meant. Once I've selected the right one, an import statement is transparently added to the top of the code (if it's not already there). Also, unused imports are automatically flagged with a warning. This means I can almost completely ignore import statements. Less to think about--very nice.
• The "refactor" and "source" menus. I can rename a symbol just by hitting Alt-Shift-R; it even renames files and suggests changes to comments accordingly. I can generate stubs for implementing an interface with a couple of clicks.
• Compile-as-you-go. Not only is compilation automatic, the IDE highlights syntactic/spelling mistakes just after you type them.

I would sure like to have these features when writing Python. I know there are some Eclipse + Python projects, but I haven't taken the time to learn whether they include the features I listed. I'll take some time to find out soon.