>>397
I would say RISC OS, a classic like OS/2, but you may need an ARM computer/emul to make it run. Think ppl got it to run on a Raspberry Pi.

As for x86 platform, a little, often overlooked one is GEOS for PC. Similar Win 3.x, nowadays it is foss, iirc. Also marketed as GEOWorks, Breadbox Ensemble a.o..

>>398
Newest is OS/2 derivate is ArcaOS, but non-free. Quite a bit modernised, so it installs on more recent PCs. Sadly idiots at IBM never released OS/2 source, could have been the better Windows... :(