ITS
ITS was the Incompatible Time Sharing system developed at MIT to run on DEC PDP-6 and PDP-10 systems. It is notable for being the primary host of MACLISP, which has nothing to do with Macintoshes but instead was named after MIT's Project MAC. ITS was not only the host for MACLISP but both of the two were used to build the first Lisp Machines, the CONS and CADR.