Unix was never a single OS, it was mostly about being POSIX compatible. In the "Unix war" everyone wanted to call themselves Unix but couldn't because of AT&T trademark so they chose different names.

- AIX is Unix from IBM - A/UX is Unix from Apple. - They’re both AT&T System V with gratuitous changes. - Then there’s HP-UX which is HP’s version of System V with gratuitous changes. - DEC calls its system ULTRIX. - DGUX is Data General’s. - And don’t forget Xenix—that’s from SCO.

So if we want to talk about the most popular POSIX compatible systems today that are free we're left with the different kinds of BSDs and Linux OS. I also mentioned MacOS in the article, in relation to CoreAudio, but decided to leave it out as its design can't be inspected and compared properly. And also it's not a very popular OS amongst my readership and the world in general.