I daily drive 9front.

The historical emergence answers here are fine as far as they go, but they're a bit like saying that Linux is an OS developed by Linus to help him learn x86.

IMO 9front is an exposé of what an OS can look like when it's written for and by hackers with a focus on simplicity. The YouTube channel adventuresin9[0] parades some nice examples of what real world usage looks like.

For example, since everything is just a file, you tunnel traffic with a simple bind mount. Since everything is just text, small regexes suffice to route mouse clicks to UI actions. Since windows are just buffers, grepping history is grepping the window's text buffer; taking a screenshot is just cp-ing the window's draw buffer.

Oh, and 9front C diverges from the standard intentionally in some nice ways: evaluation of function args is sequenced, anonymous unions like

    struct {
      int a;
      union {
        uint i;
        int j;
      };
    }

do what you want, the libc is a lot leaner and cleaner, etc.

[0]:https://www.youtube.com/channel/UC7qFfPYl0t8Cq7auyblZqxA