At the moment, about 90% of OS's available are based upon the BSD 4.4 standard. That includes all unix and linux OS's, and several other posix compliant OSs. It includes Mac OSX, as well, which is actually BSD Unix. It includes every functional supercomputer in north america that I've heard of save 1, which was, last I heard, running VMS.
Now, 75% of the installed userbase worldwide, counting all OS based products including cellphones and ebook readers, and palmtops, runs MS Windows. Less than 1% use something other than a posix, palm or windows OS. (Most cellphones and readers run Linux; one reader runs WinCE, as do several phones, and a large chunk of PDAs, and almost all the rest of the PDAs run PalmOS.) 15 years ago, VMS was still holding a significant sliver, but Unix and Linux installs provide the same functionality with less overhead and lower upkeep costs, plus access to more software. You can still get a VMS machine.... but in general, that's only done as a legacy software issue.
Most servers run Linux. Most routers run Linux. Lots of different flavors of Unix and Linux, but they all adhere to a collection of standards enforced by the Trademark licenses. Without linux, most of us couldn't afford the internet.
20 years ago, several OS's had significant shares: Windows 3.11, Windows NT, MS DOS, Mac OS 7, OS2, Unix, Amiga, Atari TOS, VMS, IRIX. 25 years ago, it was MS Dos, Windows 3, Mac OS 1, Apple DOS 3.3, Apple DOS 8, Atari TOS, Unix, VMS, Sinclair OS, Commodore Vic20/C64 COS, Apple Pascal (OS and Programming environment in one), CP/M.
the BSD 4.4 lite open source release really set the standards for the internet.
And every specialized supercomputer I've heard of is running Unix or Linux... sometimes custom builds thereof, but still, a posix compliant unix or linux.