[Oxidation]

Indeed, Linux is probably the most famous. It's trademarked because someone else tried to trademark "Linux" and then shake down vendors for 10%. It went to court, the trademark troll lost and the trademark itself was assigned to Torvalds.

It's now administered by the Linux Mark Institute.

https://en.wikipedia.org/wiki/Linux_Mark_Institute

[haunter]

That made me wonder, UNIX is also trademarked https://unix.org/trademark.html

And there are handful of OSes registered as UNIX Certified https://www.opengroup.org/openbrand/register/

Yet there is no single "UNIX OS"

[Oxidation]

The AT&T Unix is probably "the" Unix, if there is one, but the name got sold eventually to The Open Group which licenses it to compliant systems.

So it's now a bit more like x86 (family of similar products that work somewhat similarly, though x86 doesn't need a license any more) rather than Pentium (a product or range from a specific company).