Apollo-11
Original Apollo 11 Guidance Computer (AGC) source code
...Developers can examine navigation routines, guidance logic, and task scheduling in an environment with no luxury features—everything is explicit and resource-aware. The repository’s structure and documentation make it approachable for modern readers despite the archaic environment. It’s frequently used in education, talks, and explorations of software history to show how a landmark mission depended on disciplined code and verification.