Cuis is an Open Source, multiplatform Smalltalk-80 system.
What sets Cuis apart from other Smalltalk systems is the focus on the original values of the Smalltalk project at Xerox PARC, and an active attitude towards system complexity: Just keep it to a minimum!
There is more to be said about Cuis, its goals and ideas. See About Cuis.
The easiest way to download and run Cuis is to download the Zip file. Double click on it to unzip. Or you can just clone this Git repo instead.
Run the appropriate start script (RunCuisOnWindows.bat, RunCuisOnLinux.sh, CuisVM.app). To start Cuis on a Mac from Terminal, run RunCuisOnMac.sh.
Note: On Mac, if you dowloaded the Zip File, you'll likely need to open a Terminal on the Cuis folder, and run ./unquarantine.sh . This is needed for macOS to allow running an application extracted from a Zip file.