Multics (Multiplexed Information and Computing Service) was an extraordinarily influential early time-sharing operating system.
Initial planning and development for Multics started in 1964. Originally it was a cooperative project led by MIT (with Fernando Corbató), along with General Electric and Bell Labs. Bell Labs dropped out in 1969, and in 1970 GE's computer business, including Multics, was taken over by Honeywell.
More on [ Multics ]

Multicians.org - Multics was the acronym for Multiplexed Information and Computing Service - a mainframe timesharing operating system begun in 1965 and used up until 2000. Although this was the immediate forerunner of the Unix operating system, it was also in many ways its antithesis.
Meta Description: [ Multics is a mainframe timesharing operating system begun in 1965 and used until 2000. It was a major influence on subsequent systems. ]
Cardiff Multics - A brief history and some photographs of the system at the University of Wales.
Meta Description: [ A look around the Cardiff Multics machine room ]
Honeywell DPS8 - Brief technical information on the DPS8 platform used to run the Multics OS, plus links.
Mainz Multics - The history and technical details of the DPS8 installation at the University of Mainz, Germany. Site includes a photo gallery of Multics hardware components.
Multics Home Page - Information on Multics, much of it from the newsgroup alt.os.multics.
Multics Psalm - A humorous prayer well-known to veteran Multicians. Scanned PNG file.
Multics Repository - A collection of applications written in PL/1, exec_com and other languagues, plus papers and other documentation [ftp].
Multics Security Evaluation: Vulnerability Analysis - The USAF's analysis into potential weaknesses and strengths of the Multics OS. Published in 1974. Requires Acrobat Reader.
Risks Digest: The End of the Multics Era - Article by Peter G. Neumann on the decommissioning of the last Multics system at the Canadian Department of National Defence in October 2000.
Seattle Weekly: Multics and Mr. Rogers - Marking the demise of the Multics OS and drawing parallels with the PBS show Mr. Rogers' Neighborhood.
Meta Description: [ The latest Kiss my ASCII columns for Seattle Weekly. ]
Slashdot: The Last Multics System Decommissioned - The announcement of the closure of the last Multics system and a brief discussion as the significance of the Operating System.
Meta Description: [ The Last Multics System Decommissioned -- article related to Unix. ]
The Creation of the Unix Operating System - The history of the development of Unix at Bell Labs, and how Multics was an important stepping stone.
Wikipedia - Multics - A brief history and description of this operating system with links to further reading.