Well, I am glad to see that RT-11, RSX-11, and OS/360 were submitted by another old fart, but as long as we're at it, here are some more operating systems (many of which I've worked on, others I know of personally):
Apple DOS - you can hardly say that the operating system on the Apple-II was a "Mac" operating system
C.mmp/Hydra - one of the (if not _the_) first multiprocessor operating systems from CMU.
C/M* - another multiprocessor operating system from CMU.
Chorus - a micro-kernel operating system from Chorus Systemes in France (further research shows that the same folks have created Jaluna-2)
K42 - an IBM open source experimental operating system
Domain/OS - the Apollo operating syste (no, not the space capsule, the company was called Apollo Computer Corporation; research shows it was previously called Aegis)
POS - the Perq Operating syste (from Three Rivers Computer Corporation, and lovingly nicknamed Piece of Shit by many)
PrimeOS - ran on Prime computers
How Many Operating Systems Can You Name?