ITS is the Incompatible Timesharing System for PDP-10's. ITS was developed at MIT after the MULTICS Operating System, in the Artifical Intellegence Lab.
ITS was one of the first OS's connected to the ARPAnet, and it was on an ITS system that the first versions of Zork were created.
Some information on installing & images can be found here.
Installing ITS on SIMH