Multics
Multics was a highly ambitious OS developed jointly by Bell Labs, General Electric, and the Massachusetts Institute of Technology. The goal was to create a computer that worked as a water or electric utility does; a central mainframe which thousands of users would have access to. Because of this, stability and security were the main focus.
Built on machines running CTSS, development of Multics began in 1964; but a fully working system was not released until 1969.
Multics was plagued by complexity from the start, and at many points became almost too expensive and difficult to continue development. Bell Labs dropped out of the project in 1969, and GE's computer business was bought by Honeywell in 1970. Multics implemented many novel ideas for the day, many of which influence OS design to this day.