My Guru Labs coworker mentioned something to me today, that our boss mentioned to him (or a class, who knows), and that is that every operating system turns to Unix eventually. Thinking about this for a second, he's right.
First, there are the obvious Unix-like operating systems such as GNU, Linux, Minix, and others. Their intention is to be as Unix-like as possible, while maintaining different goals. Of course, these started out as Unix clones, and will stay that route, so not much to say here.
Then, there was the Apple Macintosh. Developed completely on its own, apart from any code base. It had a loyal fan base, and good features. When Steve Jobs returned to Apple, the first thing to do, was refresh the operating system. Based on Nextstep along came OS X. Built on a hybrid Unix kernel and pulling many implementations from FreeBSD, it's as Unix as Unix gets, with a pretty GUI. Apple even prides itself in advertising that OS X is Unix under the hood. Not to mention, as a company, Apple had its hands in a Unix derivative, specifically A/UX.
Some of you may remember BeOS. It was the little operating system that could. Technically, it was not a Unix-derivative, and worked hard to stand on its own. Yet, it had many features that to the lay user, would make him think he's on a Unix machine. The BASH shell was ported to BeOS and it remained POSIX compliant. It also used a single-rooted Unix-like directory structure, with a 64-bit filesystem and preemptive multitasking, uncompromising characteristics on any standard Unix-clone.
Finally, Microsoft Windows. Just as with Apple Computer, Microsoft had its very own Unix called Xenix. I'll spare you the history lesson. Suffice it to say, Microsoft put a lot of money and research into Xenix, until they sold it to SCO, and decided on OS/2 with IBM. After abandoning OS/2, they placed their focus with Windows NT. First and foremost? Take the TCP/IP networking stack from BSD Unix, and incorporate it internally. The next major move towards Unix was PowerShell, a Unix-like, POSIX compatible .NET framework shell for Windows "power users". Of course, those on Vista are becoming familiar with User Account Control, a mandatory access control system, seen on Unix-like systems for a bit now.