#5 GNU Emacs

Came into existence: 1984

Emacs and its derivatives are a family of text editors that are characterized by their extensibility. GNU Emacs is an extensible, customizable 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.

#4 X Window System

Came into existence: 1983

At the same time that Emacs and gcc started to roll, others at MIT were working on the X Window System. The X Window System is a computer software system and network protocol that provides a basis for graphical user interfaces (GUIs) and rich input device capability for networked computers.  No one knew it at the time that X Window would eventually become the basis for all important Linux and Unix interfaces, and the foundation for the Mac OS X interface.

Read also: 10 Ways How Microsoft Is Actually Falling Apart