It was a heavily graphical development tool and runtime with a very approachable scripting language. At one point, Apple included it with new Macs. At another point, they tried to charge money for it, which I think pretty much ruined it.

Nothing I've seen since really filled the role it did. There were, of course significant limits, but it was universal (if you had a Mac at a certain point in time) and had an amazingly low barrier to entry.