The Incompatible Timesharing System (ITS) was an OperatingSystem used at MIT from the late 60's to the mid 80's.
It's noteworthy for being a very open system, coded almost entirely in assembly and developed with the anti-pattern in mind that ComputerSecurity ultimately hinders user freedom. A contemporary project at MIT was a system with an extensive security model; this system was called Multics, which although also influential (having heavily inspired the creation of Unix) was antithetical to the philosophy of the HackerPersons who created ITS.
A lot of early pioneering work in ArtificialIntelligence was done on ITS, with one of the first successful LispLanguage systems, as well as the first compilers for SchemeLisp and the venerable predecessor to GnuEmacs.
It can be said that ITS is to an assembly hacker what a LispMachine is to a Lisp hacker--in fact it was the limitations of running large programs on a time-shared PDP-10 that eventually spurred on the development of the Lisp machines.
Roll your own: https://github.com/PDP-10/its