Many other "purer" unix operating systems still exist today, and are very much in use. The three BSD's and Solaris are some examples. As for UNIX with the capitol letters, that refers to the SCO owned operating system. I don't have much idea about it.
Linux is generally much better for home/workstation use, however, the others have their strengths for many other purposes. FreeBSD and OpenBSD are very secure OS's, with emphasis on maintaining the entire OS, including the kernel, shell, libraries and utilities, together as one unit. Linux on the other hand is just a kernel, the core of the operating system, and it leaves the choice of utilities to the user. The GNU software is what is used by near about every distribution, that is why they are refered to as GNU/Linux distributions.