Few people nowadays would have heard of an operating system called Multics, but almost everyone uses operating systems whose fundamental features first appeared in Multics.

If the name sounds a bit like Unix or Linux, that's no coincidence.   Ken Thompson and Dennis Ritchie wanted to write a computer game, but got kicked off Multics as each run of the game on the GE-645 timeshare mainframe cost $65 (in 1969 dollars!)   So they found an old minicomputer sitting in the corner, and wrote this cut down version of Multics to which they assigned a pun name: Unics.   Castrated Multics.   Unics became Unix.

The core principles of Multics were high availability and security.   It was meant to be the "utility " operating system, running power grids, telecommunications carriers and other essential services.   General Electric and Honeywell were the first commercial supporters.   MIT shut down it's last Multics server in 1988, but today they've announced that the source code and documentation for Multics is now freely available for download here:

http://web.mit.edu/multics-history/

More detail on Multics can be found at Wikipedia , and the Bitsavers site has a documentation section for Multics in their Honeywell archive.