I have used UNIX for well over 20 years (showing my age there), including variants such as: Linux, Mac OS X, Sun Solaris, SGI IRIX, HP-UX, IBM AIX, BSD and the UNIX-like Domain-OS, (given in no particular order of preference).
Links:
[1] http://www.unix.org/
[2] http://www.unix.org/what_is_unix/flavors_of_unix.html
[3] http://en.wikipedia.org/wiki/Unix