Significant development of Emacs, a highly customizable and extensible text editor, at the MIT AI Lab. His version was initially called 'Mocklisp'.

Creation of GNU Emacs, a free software implementation of the Emacs editor, which became a cornerstone of the GNU project. He wrote much of it in C and Lisp.

Initiates the development of the GNU Compiler Collection (GCC), a free software compiler system supporting multiple programming languages. This was crucial for building a fully free operating system.

Contributes to the development of the GNU Debugger (GDB), a free software debugger, essential for debugging and developing software applications.

Continues to maintain and contribute to various GNU software projects, ensuring their continued development and availability as free software.