Multics
Multics is the "great" OS (written in PL/I) whose design began in the late 60's, co-written by MIT, AT&T Bell Labs and GE, that was meant to become the be-all- end-all of OS technology by having just every (un)imaginable feature, and that was supplanted by Unix, despite the fact that Multics was much better(?) designed, because Unix' simple and stubborn design was much more adapted to the very resource-poor architectures of the time. Ken Thompson began UNIX after AT&T Bell Labs pulled out of the Multics project in 1969. The official story is that it was a research system, but we all know that it was really so he could port SpaceWar to the PDP-7. Eventually, Multics was aquired by Honeywell, and supported by them. The last known Multics mainframe was taken offline in October, 2000.This page is linked from: PL/I