Emacs
From Aventine Solutions
A classic editor, coming from the original Unix world in the 70's and the arch enemy of another editor and big geek favorite, vi the latter which is almost always (in my experience) included in the base Unix installation (Emacs on the other hand, is almost never there from the start). Developed with an unintelligible functional language I could never learn, Lisp, the open source community has spawned literally hundred's of versions of Emacs thus there are several ways to equip Windows and MacOSX with this old friend. If someone asks me these days should I learn Emacs, I usually discourage them because of the absurd use of the control key, but for brute-strength editing finesse I keep returning to it.
If you're still curious about this friendly monster, then a good place to start is the GNU Free Software Foundation ...
[edit] http://www.gnu.org/software/emacs/emacs.html
For Windows users that want to dig in, I found the NTEmacs port the best
