This is a list of operating systems.

The first operating systems
CTSS (The Compatible TimeShare System, made at MIT by Corbato and others)
Incompatible Timesharing System (The Incompatible Timeshare System, made at MIT)
Multics (project by Bell Labs, GE, and MIT)
Master programme for Leo Computers, Leo III in 1962
THE operating system (by Dijkstra and others)

The first proprietary computer operating systems
Apple Computer (first version was firmware with Integer BASIC; later versions had Microsoft BASIC)
Business Operating System (BOS) - cross platform, text-based
Commodore PET, Commodore 64, and Commodore VIC-20,
First IBM-PC had 3 OSes to start, UCSD P-system, CPM-86, PC-DOS
Flex (by Technical Systems Consultants for Motorola 6800 based computers: SWTPC, Tano, Smoke Signal Broadcasting, Gimix, etc.)
FLEX9 (by TSC for Motorola 6809 based micros)
mini-FLEX (by TSC for 5.25" disks on 6800 based machines)
Sinclair Micro and QX, etc.
TRS-DOS, ROM OS's (largely Microsoft BASIC implementations with file system extensions)
TI99-4

Unix-like and other POSIX-ready systems
AIX (Unix from IBM)
Amoeba (research OS by Andrew S. Tanenbaum)
AtheOS (continued with the Syllable code-fork)
A/UX (Unix Apple OS from start of 1990s)
BSD (Berkeley Software Distribution, a type of Unix)
Cromix (Unix-emulating OS from Cromemco)
Coherent (Unix-emulating OS from Mark Williams Co.)
DNIX
Digital UNIX (later, HP's Tru64 for Digital's AXP 64-bit computers)
FreeBSD (Version of abandoned BSD with free source code)
GNU/Hurd
Haiku (operating system)
HP-UX from HP
Idris from Whitesmiths
IRIX from SGI
LainOS (FreeBSD project, a tried version of "Navi" GUI from Serial Experiments Lain)
Linux (free Unix-like kernel)
Minix (educational OS by Andrew S. Tanenbaum in the Netherlands)
NetBSD (post-CSRG open source type of BSD)
NeXTSTEP (foundation for OS X)
OS-9 (not to be confused with Mac OS 9)
OS-9/68k
OS-9000 (OS-9 written in C)
OS/360
OSF/1
OS X from Apple Inc.
OpenBSD (post-CSRG open source type of BSD)
OPENSTEP NeXTSTEP on Intel x86, HP PA-RISC, and different architectures.
Plan 9 (by Bell Labs)
QNX (POSIX, microkernel OS)
Rhapsody
RiscOS
SCO UNIX (from SCO, purchased by Caldera)
Solaris from Sun Microsystems
SunOS from Sun Microsystems (became Solaris)
System V (a release of AT&T Unix, 'SVr4' was the 4th minor release)
UNIX (OS developed at Bell Labs in 1970 first by Ken Thompson)
UNIflex (Unix emulating OS by TSC)
Ultrix (DEC's first version of Unix for VAX, PDP-11, MIPS, and Decstation computers)
UniCOS from Cray
Xenix (Microsoft's version of Unix)
z/OS (IBM mainframe OS)

Generic/commodity, non-UNIX, and other
AOS, now called Bluebottle(update to Oberon operating system)
AROS (Amiga Research Operating System)
Bluebottle (see AOS)
BS2000 by Siemens AG
Control Program/Monitor-80 (CPM operating system)
CP/M-86 (by Digital Research)
DESQView (windowing GUI for MS-DOS,1985)
DR-DOS (MS-DOS compatible OS from Digital Research)
FLEX9 (by TSC)
FreeDOS (open source, free MS-DOS alternative)
GEM (GUI for MS-DOS by Digital Research)
GEOS
MS-DOS (Microsoft purchased from Seattle Computer)
MorphOS (by Genesi)
NetWare (by Novell)
NeXTStep (Foundation for OS X)
PC-DOS (IBM's version of MS-DOS)
Pick
Plan 9, Inferno (networked OS by Bell Labs)
Primos by Prime Computer
Mach (kernel by CMU, used in NeXTStep and OS X)
MP/M-80 (by Digital Research)
NewOS
Oberon operating system (developed at ETH-Zürich by Niklaus Wirth)
OS/2 Windows/MS-DOS compatible operating system made in a Microsoft and IBM project, but later Microsoft stopped when they wanted to work on Windows NT more; which was better than early Windows versions and MS-DOS.
OS-9 (Unix emulating OS from Microware)
OS-9/68k (Unix emulating OS from Microware)
OS-9000 (portable Unix emulating OS from Microware)
SSB-DOS (by TSC for Smoke Signal Broadcasting; a variant of FLEX)
TripOS
TUNES
QDOS (made at Seattle Computer Products by Tim Paterson for new Intel 808x CPUs. Given to Microsoft—soon became MS-DOS)
UCSD P-system (OS by a student project at the University of California, San Diego; directed by Prof Ken Bowles)
VisiOn (first GUI for early PC machines, not successful)
Visopsys (hobby OS for PCs)
VME by International Computers Limited (ICL)
Randolf by United Computer inc,
Vision (first GUI for early PC machines, not successful, by Microware)

Hobby OS
Operating systems written for a hobby.
ReactOS (an open source OS that runs Windows NT applications natively)
Panalix
MenuetOS
SkyOS

Proprietary

Acorn
Arthur
ARX
RISC OS (was later open sourced under a fully free/open source license)
RISCiX

Amiga
AmigaOS
AmigaOS 4

Atari ST
MultiTOS
MiNT
TOS

Apple
Apple DOS
A/UX
Darwin
GS/OS
iOS
Macintosh/Mac OS classic
Mac OS X
Newton OS
ProDOS

Be Incorporated
BeOS
BeIA
Zeta

Digital/Compaq/HP
AIS
ITS (for the PDP-6 and PDP-10)
OS-8
RSTS/E (ran on several machines, chiefly PDP-11s)
RSX-11 (multi-user, multitasking OS for PDP-11s)
RT-11 (single user OS for PDP-11)
TENEX (from BBN)
TOPS-10 (for the PDP-10)
TOPS-20 (for the PDP-10)
VMS (by DEC for the VAX mini-computer type; later renamed OpenVMS)
WAITS

IBM
AIX (a version of Unix)
ALCS
Basic Operating System (first system released for the System 360, as an interim)
DOS/VSE
MFT (later called OV/VS1)
MVS (latest variant of MVT)
MVT (later called OV/VS2)
OS/2
OS/360 (first OS planned for the System 360 architecture)
OS/390
OS/400
PC-DOS (IBM's version of DOS)
SVS
TPF
VM/CMS
z/OS

Microsoft
Xenix
MS-DOS
Microsoft Windows
Windows 1.0x (Mostly text interface organised into windows, because graphical hardware was expensive at the time)
Windows 2.X
Windows 3.X
Windows NT
Windows NT 4.0
Windows 9x
Windows Me
Windows 2000
Windows XP (based on code of Windows 2000, was first "non-business" version of Windows NT)
Windows Server 2003
Windows Vista
Windows Server 2008
Windows 7
Windows Server 2008 R2
Windows 8
Windows Server 2012
Windows 8.1
Windows Server 2012 R2
Windows 10
Windows Server 2016
Windows Server 2019
Xenix

Personal digital assistants (PDAs)
EPOC, today is named Symbian OS
iOS from Apple
Newton OS from Apple, for  Apple Newton MessagePad
Palm OS from Palm Inc
Pocket PC from Microsoft
Windows CE (Windows Compact Edition, from Microsoft)

Microcontroller, embedded
Little operating systems that run on small devices.
Contiki
INTEGRITY
ITRON
Nucleus RTOS
OSEK
QNX
ThreadX
TRON OS by Ken Sakamura
VxWorks
µCLinux
eCos

Fictional operating systems
Operating systems that have only appeared in fiction or as jokes.
Lcars - From Star Trek
ALTIMIT OS - From .hack
Digitronix - From The Hacker Files
Hyper OS - From the movie Patlabor
Penix - Pun of Microsoft Xenix with an inappropriate word
Wheatonix - April fool's joke.
GLADOS - Genetic Lifeform and Disk Operating System

Related pages
Computer software
Disk operating system

Operating systems
Computer-related lists