ITS is a timesharing system created at MIT's Artificial Intelligence Laboratory in 1967 for their PDP-6 computer. It was soon moved to the newer PDP-10; for most of its life ITS ran on four of those machines. ITS is known for it's lack of security (no passwords, no file permissions), using a debugger as the login shell, and pioneering many famous programs.
The SDF ITS is at https://hactrn.org/ and is running on a KA10 emulator with an attached PDP-11 for serving Knight TV raster graphics terminals.
Some basic tutorials:
More information can be found by running INFO, and on GitHub.
Suggested topics to explore: