XEmacs - http://www.xemacs.org
Unix, Windows, probably others. If what you want is a programmer's
editor, and not a text
editor, you want something with this much power. Vi would also work, but my fingers speak emacs, so it works better for me. Cool feature I use the most: macro recording, playback, editing. You can customize pretty much everything about the editor. Program your own extensions if you feel like learning Lisp, and can't get the features you need in available extensions (not all that likely, really). There's even a Tetris mode for when you need a little break from work...
No matter which editor you choose, spend some time getting to know how to use it. Then spend more time learning how to abuse it. If you live in an editor window most of your day (like me), every trick you learn makes you that much more productive.