NetLogo

Uri Wilensky built it in 1999 at Northwestern as a successor to StarLogo, with Logo syntax underneath. The world consists of turtles that move, patches that are the squares they move over, links between turtles, and an observer that sees everything. A few lines of code produce a flock that behaves like birds.

The strength is not the language but the Models Library: hundreds of ready-made, documented models — Wolf Sheep Predation, Schelling's segregation model, epidemic spread, traffic jams — that run and can be tinkered with immediately. That is why it turns up heavily in teaching and in published research across ecology, epidemiology and the social sciences, well outside the population that would otherwise write code at all.