Emacs
termprogrammingops culture
The extensible editor from the GNU project, closer to a programmable environment that happens to edit text.
Nearly all of Emacs is written in its own Lisp dialect and can be redefined while running, which is what people mean when they joke that it is a fine operating system lacking only a decent editor. Its rivalry with vi is the oldest running joke in the field, the editor war, and both sides are correct: one optimizes for editing keystrokes, the other for reshaping the tool itself.
Also known as: GNU Emacs, editor war