GNU Hurd
Stallman was responsible for contributing many necessary tools, including a text editor (Emacs), compiler (GCC), debugger (GNU Debugger), and a build automator (GNU make). The notable omission was a kernel. In 1990, members of the GNU project began using Carnegie Mellon's Mach microkernel in a project called GNU Hurd, which has yet to achieve the maturity level required for full POSIX compliance.