Emacs turned text editing into a programmable environment by putting commands, key bindings, modes and user extensions behind an interpreter that could be changed while the editor was running.
Richard Stallman's GDB made source-level debugging part of the GNU system and evolved into a portable, community-maintained debugger for many languages, architectures and remote targets.
Richard Stallman's GNU C Compiler made a high-quality optimizing compiler available as free software in 1987 and grew into the multi-language GNU Compiler Collection that underpins major open systems.
The GNU Project turned software freedom from a principle into a systems-building program: create a complete Unix-compatible environment that users could study, modify, share and improve.