If you're going to list A/UX as a discrete operating system, then you should list every flavor of UNIX ever. sudarkoff got a bunch, but also there are Irix (from Silicon Graphics), SunOS, Solaris, NetBSD, OpenBSD, GNU/Hurd, SCO Unix, AIX (IBM's UNIX), DG/UX (not sure whose this was), and probably a bunch more. I haven't seen any mention of Cray machines yet -- I think they usually run a custom flavor of UNIX. Oh, and what about Apple's early OSes, like their DOS 3.3 that ran on the ][e, also ProDOS, and that one that only ran on the IIgs. And the Lisa OS. Dude, this could go on for a while ...
How Many Operating Systems Can You Name?