In case you're also interested in niche operating systems and are also disenthused by the fact that most of them are hot garbage, I will provide my list of interesting operating systems projects:
- 9front (<3)- RedoxOS- ReactOS- ToaruOS- Haiku*- Sortix- Genode*- SerenityOS*
* Caveat: heavily involve C++ for some fucking reason
@sir are *BSDs not niche for you - at least more exotic ones, such as DragonflyBSD? (I won't call the bigger ones: FreeBSD, OpenBSD, NetBSD niche, but the rest?)
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