Richard Stallman
Richard Stallman launched the GNU Project in 1983 to build a complete, Unix-compatible operating system made entirely of free software, and founded the Free Software Foundation in 1985. He also authored the GNU General Public License (GPL), the 'copyleft' license that guarantees users the freedom to run, study, share, and modify software, and which underpins a huge portion of the open-source world.
He personally wrote several of the most important GNU tools, including the GNU Compiler Collection (GCC), the GNU Emacs editor, and the GNU Debugger (GDB). The GNU userland — compiler, C library, shell, and core utilities — is what surrounds the Linux kernel to make a usable system, which is why Stallman insists the combination be called GNU/Linux rather than just Linux.
For more than forty years he has been an uncompromising activist for software freedom and user rights, a stance that has made him one of the most influential and most polarizing figures in computing.