The version 3 with the Gadgets framework was quite cool.

The only modern OS environments that come close to these experiences are Windows with .NET/VS/Powershell and OS X with Playgrounds, the OS X Frameworks and Applescript.

Don't get me wrong, it is great what they are doing and I for one enjoy Notebooks, Playgrounds, modern IDEs, tooling that allows for both for JIT and AOT, while offering safety.

Just a bit nostalgic how things could have been different.

For example, when Google brought out ChromeOS and Dart, I thought there was a perfect opportunity to have a similar experience, but that is not how it turned out to be.

EDIT: I guess I should expand a bit on that answer.

Take Oberon, which like Java has the misfortune of using the same name for the language and the respective OS.

It was based on Niklaus Wirth experience on using the Xerox Star system with Mesa/Cedar environment.

The original Native Oberon used as only executable format shared libraries. Depending on the OS variant, they were either fully AOT, or JITted on load.

There was a convention on procedures that could be made visible to the REPL or mouse commands and what type of input they would act upon.

So you could either do module.procedure parameters at the CLI, select some text at a given open window or embedded object (think OLE) and select a module.procedure at the toolbox of loaded modules.

Very simple set of primitives exposing the whole OS, both CLI window and other GUI elements as programmable elements.

You see a bit of this way of working on the ACME editor used in Plan 9, specially later on Inferno with Limbo.