Emacs

Emacs is the extensible, customizable, self-documenting real-time display editor.

[from Editing MACroS]

To those who don't appreciate vi.

A programmable text editor with an entire Lisp system inside it.

To others ‘Eight Megabytes And Constantly Swapping’ or Esc Meta Alt Ctrl Shift

EMACS entry in the Jargon File

GNU Emacs

GNU Emacs FAQ For Windows

EmacsWiki

XEmacs