1. Incompatible time-sharing System
An influential but highly idiosyncratic operating system written for the PDP-6 and PDP-10 at MIT and long used at the MIT AI Lab. Much AI-hacker jargon derives from ITS folklore, and to have been "an ITS hacker" qualifies one instantly as an old-timer of the most venerable sort. ITS pioneered many important innovations, including transparent file sharing between machines and terminal-independent I/O. After about 1982, most actual work was shifted to newer machines, with the remaining ITS boxes run essentially as a hobby and service to the hacker community. The shutdown of the lab's last ITS machine in May 1990 marked the end of an era and sent old-time hackers into mourning nationwide (see high moby). The Royal Institute of Technology in Sweden is maintaining one "live" ITS site at its computer museum (right next to the only TOPS-10 system still on the Internet), so ITS is still alleged to hold the record for OS in longest continuous use (however, WAITS is a credible rival for this palm).
2. A mythical image of operating system perfection worshiped by a bizarre, fervent retro-cult of old-time hackers and ex-users (see troglodyte). ITS worshipers manage somehow to continue believing that an OS maintained by assembly language hand-hacking that supported only monocase 6-character filenames in one directory per account remains superior to today's state of commercial art (their venom against Unix is particularly intense).