ITS and the Logo programming language
ITS was host to a lot of Logo development. There are many implementations available:
PDP-11 Logo was the reference implementation. It ran on a PDP-11/45 with special display hardware added. There were older vector displays by Tom Knight, and newer raster displays by Ron Lebel (mostly same as the Knight TV hardware).
At first PDP-11 Logo ran on the metal, but later a timesharing system was developed. It was called SITS, or Small ITS. The initial design was similar to ITS, but later development took inspiration from MIT's PDP-1X timesharing system.
A minimal Little SITS emulator was used to run PDP-11 Logo on the General Turtle LSI-11 processor.
Hal Abelson et al wrote an interesting game using the vector displays, called Dazzle Dart. Two teams of two plays against each other, and the objective is to shine a ray of light in the opposing team's goal. The ray must reflect at least once for a goal to be scored, making it possibly the first game with ray tracing.