Objective-Smalltalk source code is available on github:
https://github.com/mpw/Objective-Smalltalk.
It requires MPWFoundation, which actually implements much of the architectural components and connectors:
https://github.com/mpw/MPWFoundation.
If you want to build the GUI components, including the Smalltalk application, you will also need DrawingContext:
https://github.com/mpw/DrawingContext.
The two frameworks are best built together in a workspace. You can use the one included in Objective-Smalltalk or use your own. Building either the Objective-Smalltalk target or one of the sample applications should build everything.
Objective-Smalltalk includes a number of sample applications
stsh
A Unix command line interpreter that can be used interactively or for script execution. In addition to the MPWFoundation and Objective-Smalltalk frameworks, this also requires Stsh.framework, which is part of the Objective-Smalltalk project and built automatically.
In order to use, the frameworks must be copied to /Library/Frameworks/ and stsh itself to /usr/local/bin/.
Smalltalk.app
A very early cut at duplicating small parts of the Smalltalk integrated image experience without actually having an image. Right now, you can open multiple workspaces and a REPL.
Apart from general exploration and programming tasks, I've also been using the Smalltalk.app together with the Slides3D framework to create presentations.
The scripts directory in the Objective-Smalltalk project includes a number of example scripts.