Emacs is distributed by the Free Software Foundation and was Richard Stallman's first step in the GNU project. Emacs is extensible - it is easy to add new functions; customisable - you can rebind keys, and modify the behaviour of existing functions; self-documenting - there is extensive on-line, context-sensitive help; and has a real-time "what you see is what you get" display. Emacs is writen in C and the higher levels are programmed in Emacs Lisp.
Emacs has an entire Lisp system inside it. It was originally written in TECO under ITS at the MIT AI lab. AI Memo 554 described it as "an advanced, self-documenting, customisable, extensible real-time display editor".
It includes facilities to view directories, run compilation subprocesses and send and receive electronic mail and Usenet news (GNUS). W3 is a web browser, the ange-ftp package provides transparent access to files on remote FTP servers. Calc is a calculator and symbolic mathematics package. There are "modes" provided to assist in editing most well-known programming languages. Most of these extra functions are configured to load automatically on first use, reducing start-up time and memory consumption. Many hackers (including Denis Howe) spend more than 80% of their tube time inside Emacs.
GNU Emacs is available for Unix, VMS, GNU/Linux, FreeBSD, NetBSD, OpenBSD, MS Windows, MS-DOS, and other systems. Emacs has been re-implemented more than 30 times. Other variants include GOSMACS, CCA Emacs, UniPress Emacs, Montgomery Emacs, and XEmacs. Jove, epsilon, and MicroEmacs are limited look-alikes.
Some Emacs versions running under window managers iconify as an overflowing kitchen sink, perhaps to suggest the one feature the editor does not (yet) include. Indeed, some hackers find Emacs too heavyweight and baroque for their taste, and expand the name as "Escape Meta Alt Control Shift" to spoof its heavy reliance on keystrokes decorated with bucky bits. Other spoof expansions include "Eight Megabytes And Constantly Swapping", "Eventually "malloc()'s All Computer Storage", and "Emacs Makes A Computer Slow" (see recursive acronym). See also vi.
FTP from your nearest GNU archive site.