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You already listed my vote - Colossus 249, the software that ran on the Apollo AGC (Apollo Guidance Computer). While keeping in mind that the AGC was cutting edge for its time and it was the first computer to use integrated circuits, back then that meant 32k of storage and 2k of memory, along with 4 registers. And yet the programmers managed to accomplish a primitive form of non-preemptive multitasking that allowed 8 simultaneous (sort of) jobs. Oh yeah, and they managed to make it work well enough for a trip to the moon. And back. Pretty amazing. There are even instructions available online on how to build your own AGC, so if you want to compare your skills to those of the NASA guys from the late ‘60s, you can give it a go.