In the history of object-oriented (OO) computer languages, Smalltalk has a special place. It wasn’t the first OO language, and it hasn’t spread as well as some others. But it was the first OO language many people encountered, and it influenced many other languages you’ve probably seen. Developed at the famed Xerox PARC, it also introduced many people to the “model/view/controller” (MVC) architecture. Cuis Smalltalk is a free and portable modern Smalltalk-80 environment. It replaces the traditional MVC framework with Morphic, but otherwise, it would be very familiar to someone who used Smalltalk in the 1990s. The project’s goal is to stay simple at the core, although you can add complexity via packages.
The software has been around for a while, but it is on a rolling release model, so the install page suggests you clone the Git repo. You can, however, get a stable release, which might be important if you were deploying applications. Running the program results in a Cuis desktop where you can open various things like workspaces, help documents, and the transcript.
Smalltalk throws some people because it takes OO to an extreme. For example, you don’t add two and three using an operator. You send the object two a message that includes another object, three. You can think of sending a message as calling a method on an object. As with virtual functions in C++, different objects can respond differently to the same request. For example, an Animal class might have a speak message that causes a cow to print “moo” and a chicken to print “cluck.”
You get used to it, and there’s a very nice class browser that lets you examine classes and what you can do with them. There are even packages to work with an Arduino over a serial port. They haven’t been updated lately, so they might (or might not) need a little work.
We’ve looked at GNU Smalltalk before. We’ve also seen a version that runs directly on a Raspberry Pi.

Will it run on a 80’s 6502 or 6809 micro? With extra banked memory? Will it run doom?
It’s Turing complete, so yes. But you might not like how fast it is in those situations.