An extensible, customizable, free/libre text editor — and more.

At its core is an interpreter for Emacs Lisp, a dialect of the Lisp programming language with extensions to support text editing.

At 200ok, we're using it for task management, for writing quotes, emails and slides, we automate automate controlling with it and even integrate it into our accounting workflow. Wherever input or output is primarily text based, Emacs might be one of the best options.