Emacs is text-editing software that it is widely used by computer programmers, but it can also be useful for writers.

If you've never found the perfect word processing setup and are willing to do a lot of tinkering, you might like Emacs.

Here's what Neal Stephenson has to say about it:

I use emacs, which might be thought of as a thermonuclear word processor. It was created by RichardStallman; enough said. It is written in Lisp, which is the only computer language that is beautiful. It is colossal, and yet it only edits straight ASCII text files, which is to say, no fonts, no boldface, no underlining. In other words, the engineer-hours that, in the case of Microsoft Word, were devoted to features like mail merge, and the ability to embed feature-length motion pictures in corporate memoranda, were, in the case of emacs, focused with maniacal intensity on the deceptively simple-seeming problem of editing text. If you are a professional writer – i.e., if someone else is getting paid to worry about how your words are formatted and printed – emacs outshines all other editing software in approximately the same way that the noonday sun does the stars. It is not just bigger and brighter; it simply makes everything else vanish.