My last font change in a code editor was to ProggyCleanTT. However, Microsoft recently made available a new font, Consolas, available for licensed users of Visual Studio 2005 (found on "Only Passionate People Win"). Consolas is a significant improvement over "Courier New" and is better than Proggy Clean. Here are some examples:

Courier New (10 pt):

Proggy Clean (12 pt):

Consolas (10 pt):

Here are some more examples, this time real code (C++) (thumbnailed each one so you can open them in a new browser tab for comparison):

Courier:

Proggy Clean:

Consolas:

If you compare Consolas and Proggy Clean, you'll see how the kerning is better in Consolas which helps readability. It's not a major improvement from Proggy Clean, but if you're a licensed user of Visual Studio, and you're tired of Courier, download Consolas setup (4.3 MB) and give it a whirl. It automatically changes the Visual Studio editor font during the installation. It's free!