If you are sticking to POSIX type OS:
Windows (1, 2, 3, 95/98/ME)
Windows NT (1, 2, 3, 4, 2000/XP, Vista/7/8/8.1, 10) - Server NT (2000, 2003, 2008, 2012, 2016)
Unix
Linux (Steam OS would be a Linux distro, so not really its own separate thing)
OSX (Unix)
More server oriented ones (actually have some AIX and lots of Solaris servers at work)
AIX
HP-UX
IRIX
Solaris
Tru64
UnixWare
QNX Neutrino
Inspur K-UX
Integrity
Chrome OS would fall under Android and iOS class mobile OS.
Basically though, these are effectively Linux and Unix as well.