Lambda the Ultimate

Since it's still awfully quiet, I thought mentioning DivaScheme would be a good idea, seeing as IDEs always get the juices flowing...

When DivaScheme is on, the most important functions of DrScheme are available through unchorded keystrokes, and the motion commands operate on sexps by default. DivaScheme also takes cares of layout concerns. It automatically maintains the white spaces, the indentation, and the balance of the parentheses.

This description does not do it justice. You can read the docs or see the movie (links on the DivaScheme site).