He designed some of the software at the heart of open source software. Most notably GCC, which is a compiler for code. A compiler takes code from human readable form into a form usable by the computer. This is required by languages like C++, but not languages like python or JavaScript. He was also largely responsible for Emacs, which is the only piece of software a computer needs in addition to a browser. Although some computers, like mine, require proprietary software like Minecraft as well.