I'll only go with things you can read the source code of: The Quake 1-3 engines. Truly groundbreaking for each one, and easily understandable by mere mortals.

I'll only go with things you can read the source code of: The Quake 1-3 engines. Truly groundbreaking for each one, and easily understandable by mere mortals. They were critical in the adoption of consumer GPUs and in network gaming. With modding and everything else, they basically created an entire industry. PostgreSQL. Amazing RDBMS that has a long history and is also very approachable by the layman. It sets new records for technical writing as well, and is in widespread use. Linux. Hugely important, full of smart and not-smart pieces, and the backbone of a lot of the computing world today. Plan 9. Maybe the best version of Unix ever made, and very approachable in both its system design and implementation. It's a future that never came to pass, but is hugely interesting. BLAS/LAPACK. Gigantic and scary piles of optimizations for numerical linear algebra, but of utterly unmatched importance (barring maybe the FFT). Everything behind control and data modeling today relies on this. Erlang's BEAM. Another very approachable VM, and one that has an amazing track record. Combined with OTP, this powers a lot of very important telecom and other systems. MUMPS. A dark wonder, but almost all healthcare interacts with a MUMPS runtime at one point or another. A sobering reminder that we as engineers can do horrible things that will stay in production for decades and maybe even kill people.