Since downthread people asked for explanations.

I know nothing about Mozilla Hubs or the Third Room. No comment or comparison to make.

This "Croquet" is a new version of a circa 20 year old project:
https://en.wikipedia.org/wiki/Croquet_Project

That Croquet was an effort to extend a then circa 25 year old project, the Smalltalk programming language,.

Smalltalk is more or less the original object oriented programming language (there was Simula before it but it has different goals). Smalltalk aimed to make it easy and fun to create GUIs and GUI-driven, 2D graphical, software.

Smalltalk largely flopped in the market although some commercial and FOSS versions survive. For instance the Eclipse IDE is an offshoot of one of these.

Some of the original Smalltalk team created Squeak, a modern colourful Smalltalk that is entirely FOSS. Squeak has been used to create lots of other things such as the children's educational programming tool Scratch.

Croquet was built on and in Squeak. Croquet extended Squeak into two new directions at once: 3D graphics, including hardware acceleration, and transparent networking.

Croquet could be used to build 3D apps and entire 3D environments, including 3D games and VR along with lots of other things.

But also, at the same time, the idea is that Croquet instances on different machines can talk over a network, using a protocol called Teatime, so that those instances are connected and can share data, live, in real time.

The idea is, as I understand it, that it makes stuff like multiuser video conferencing so trivial it's basically a no-code exercise.

It is trivial to build something like Zoom or Teams or Google Meet: you draw a room, place some picture frames on the wall, connect each one to the mic and camera of the local machine, and boom, you have a video conference in 3D, without coding.

It is meant to make much more complicated ambitious stuff easy. It extends Smalltalk and makes it able to do 3D as easily as Smalltalk did 2D stuff at Xerox PARC in the 1970s, and it makes it all network-transparent, so connecting instances is as easy as coordinating two Smalltalk windows to share data: literally a fun game for schoolchildren.

Xerox PARC's Smalltalk is the 1970s tech that was the original inspiration for the Apple Lisa in 1982, and a couple of years later, the cheap cut-down version, the original Apple Macintosh in 1984. That inspired Microsoft Windows, and Windows created an industry of cheap 32-bit PCs with integrated graphics and sound which is where Linux developed and thrived.

Croquet, in 2004, took this 1970s tech that predated (and caused!) the microcomputer boom and put it on the Internet with hardware rendered 3D.

Smalltalk is the original language that popularised OOPS and so which led to C++ and Java and Python and C# and .NET and Scala and Rust and Go and all that. It is also the language which inspired the Mac and GUIs... all GUIs.

Smalltalk also directly inspired later OOPS languages, such as Self. Self subsumes Smalltalk objects into "prototypes", using a Smalltalk-like syntax.

Self later inspired Javascript, which replaces the Smalltalk-like syntax with C-type curly-brackets syntax.

Now, OpenCroquet moves Croquet off Squeak Smalltalk and into Javascript instead. It is Croquet, ported from Squeak to Javascript, so it runs inside the browser.