Multics
Time-sharing OS from MIT + Bell Labs + GE 1964–2000. Ambitious, too complex, drove the creation of Unix when Bell Labs dropped out.
Pioneered: hierarchical file system, dynamic linking, ring protection, multi-user multi-process. Written in PL/I (rare) — inspired Bjarne Stroustrup's choice to write C++. The last Multics system was shut down in 2000 (Canada Department of National Defence). Ken Thompson + Dennis Ritchie wrote Unics ("Uniplexed Information and Computing Service") as a reaction to Multics's complexity → became Unix.