Several important programming languages and systems were developed on ITS, including MacLisp (the precursor of Zetalisp and Common Lisp), Microplanner (implemented in MacLisp), MDL (which became the basis of Infocom's programming environment), and Scheme.
Multics Emacs was an implementation of the Emacs text editor written in Maclisp by Bernard Greenberg at Honeywell's Cambridge Information Systems Lab.
Note that the Franz LISP dialect was the immediate, portable successor to the ITS version of Maclisp and is perhaps the closest thing to the LISP in the Steven Levy book Hackers as is practical to operate.