Computers

Operating SystemUnix and BSDUNIX System V, AIX, HP-UX, Solaris (SunOS), IRIX, List of BSD operating systems 
GNU/Linux
List of Linux distributions, Comparison of Linux distributions
Microsoft Windows
Windows 95, Windows 98, Windows NT, Windows 2000, Windows XP, Windows Vista, Windows CE, Windows 7, Windows 2008, Windows Mobile,
DOS
86-DOS (QDOS), PC-DOS, MS-DOS, FreeDOS, DR DOS,
Mac OS
Mac OS classic, Mac OS X, X86
Embedded and real-time
Embedded Linux, FreeBSD, FreeRTOS, Inferno (distributed OS, Bell Labs), LynxOS, pSOS, Nucleus, CMX, uC/OS, eCos, ...
Experimental
GalAX, Amoeba, Oberon/Bluebottle, Plan 9 from Bell Labs