Croquet is a 3D virtual world system in Squeak.
A commercial variant is used by 3DICC to implent Immersive Terf, a large scale virtual conference system that allows you to 'be there now'.
Croquet has already a long history and I thinking, that it is quit important to keep information for all development stages, it is passed through also. Every Croquet version based on Squeak VM/Image pair has it's own unique features worth of exploration.
Also, there are lot of videos on David A. Smith's YouTube channel with Croquet demos from the "future" (https://croquet.io/), now and the past, like https://www.youtube.com/watch?v=mOZPW8l85eI\
To edit:
There is a commercial version of Croquet called Immersive Terf ().