I like Plan 9 from Bell Labs, a successor to Unix. I often say discovering Plan 9 is like discovering Linux as a Windows user for the first time, although I'm pretty sure I heard this somewhere else.
Explaining Plan 9 here would take too much effort, especially when people have already done it better than me.
I encourage you to check out some of the following:
I currently have 9front installed on my main system and boot to it sometimes. I also have various VMs and have experimented with a "world-wide" Plan 9 grid before (Most of these experiments turned out to be pretty useless)
Fact: This page was mostly written on 9front
HTML takes up a lot of screenspace
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Works:
TODO I guess