I developed a (personal) Squeak application a few decades ago and to this day it stands out as a novel software development experience that I'm very glad I did. I highly recommend everyone even remotely interested in Smalltalk read the classic "Design Principles Behind Smalltalk" [0]

Perhaps the most immediately shocking feature of Squeak is the "world" which relates to the principle:

> Operating System: An operating system is a collection of things that don't fit into a language. There shouldn't be one.

This means all Squeak programs live in their own, entirely Squeak based, virtual machine. This was, understandably, off putting to many devs since you can't bring any of your local tooling with you, but it had some interesting consequences. For starters, way back in the early 2000s, you could keep your Squeak image on a thumb drive and bring your entire dev environment with you to not only different computers, but different OSes! Then, in the Squeak window system, you could view the source of any arbitrary window or part of the gui.

Squeak, despite the small community, had some really novel software at the time. Monticello was a dvcs that predated git! There were also a proper object graph database, GemStone, that could be used for object persistence that, at least from an interface level, still beats any ORM we have today. There was also a feature that allowed method lookup by putting in the inputs and expected outputs (I still haven't seen anything like this).

In general learning about the history of Smalltalk interactively really drove home how incredibly novel of a system is was, and still remains in some ways today.

0. https://www.cs.virginia.edu/~evans/cs655/readings/smalltalk....