I'd say thousands and perhaps millions unless you qualify what you count as an OS more specifically. Many have been written as school projects, many run on obscure or even proprietary hardware platforms. Do you cound RehHat Linux as different from Slackware, is FreeBSD different from OpenBSD or NetBSD, what about OS's that run on intel hardware but are in alpha stage of development like reactos, what about abandoned projects, etc....need clarification on the question, please. I'm sure we can come up w/ loads and loads of answers ;) ☮

as per my earlier comment: here's a partial list before I bored: Arthur ARX MOS (on the BBC Micro and BBC Master) RISC OS RISC iX (based on 4.3BSD) AmigaOS Amiga Unix, a.k.a. Amix AEGIS/Domain/OS One of the first network-based systems. Ran on Apollo/Domain hardware. Later bought by Hewlett-Packard. Apple DOS ProDOS GS/OS SOS (Sophisticated Operating System) Lisa OS Mac OS * System Software 1 * System Software 2 * System Software 3 * System Software 4 * System Software 5 * System Software 6 * System 7 (code-named "Big Bang") * Mac OS 8 * Mac OS 9 A/UX MkLinux Mac OS X v10.0 (aka Mac OS X 10.0 "Cheetah") Mac OS X v10.1 (aka Mac OS X 10.1 "Puma") Mac OS X v10.2 (aka Mac OS X 10.2 "Jaguar") Mac OS X v10.3 (aka Mac OS X 10.3 "Panther") Mac OS X v10.4 (aka Mac OS X 10.4 "Tiger") Mac OS X v10.5 (aka Mac OS X 10.5 "Leopard") Darwin (open source underpinnings of MacOS X, based on FreeBSD and NextStep) Atari DOS (for 8-bit computers) Atari TOS (Tramiel Operating System), named after Jack Tramiel, who once owned Commodore Computers. Atari MultiTOS BTOS MCP (Burroughs Large Systems) CTOS BeOS BeIA ZETA (successor of BeOS developed by yellowTAB and now sold by Magnussoft) OS/8 ITS (for the PDP-6 and PDP-10) MPE (from HP) TOPS-10 (for the PDP-10) WAITS TENEX (from BBN) TOPS-20 (for the PDP-10) RSTS/E (multi-user time-sharing OS for PDP-11s) RSX-11 (multiuser, multitasking OS for PDP-11s) RT-11 (single user OS for PDP-11) VMS (originally by DEC, now by HP) for the VAX mini-computer range, Alpha and Intel Itanium 2; later renamed OpenVMS) Domain/OS (originally Aegis, from Apollo Computer who were bought by HP) RTE HP's Real Time Executive (ran on the HP 1000) TSB HP's Time Share Basic (yes, it was an operating system, ran on the HP 2000 series) Digital UNIX (derived from OSF/1, became HP's Tru64 UNIX) HP-UX Ultrix NonStop Kernel (Originally from Tandem Computers for their line of fault-tolerant platforms; originally called Guardian). It supports concurrent execution of: Guardian OSS (POSIX-compliant Open System Services) INTEGRITY Reliable Operating system INTEGRITY-178B A DO-178B certified version of INTEGRITY. µ-velOSity A lightweight microkernel. MPE Multi-programming Executive; ran on HP3000 mini-computers. iRMX real-time operating system originally created to support the Intel 8080 and 8086 processor families in embedded applications IBM 7090/94 IBSYS SYSTEM 1400/1800 IJMON A Bootable serial I/O monitor for loading programs. BOS/360 Early interim version of DOS/360, briefly available at a few Alpha & Beta System 360 sites. TOS/360 Similar to BOS above and more fleeting, able to boot and run from 2x00 series tape drives. DOS/360 Disk Operating System. First commonly available OS for System/360 due to problems in the OS/360 Project. Multi-programming system with up to 3 partitions. DOS/360/RJE DOS/360 with a control program extension that provided for the monitoring of Remote Job Entry hardware (Card Reader & Printer) connected by dedicated phone lines. DOS/VSE First DOS offered on System/370 systems, provided Virtual Storage Extensions, and SNA. Still had fixed size processing partitions, but up to 14 partitions. DOS/VSE/ESA DOS/VSE extended virtual memory support to 32 bit addresses (Extended System Architecture). z/VSE Latest version of the four decades old DOS lineage. Now supports 64 bit addresses, Multiprocessing, Multiprogramming, SNA, TCP/IP, and some virtual machine features in support of Linux workloads. (All DOS ref. IBM website) OS/360 First official OS targeted for the System/360 architecture, saw customer installations of the following variations: PCP Primary Control Program, a kernel and a ground breaking automatic space allocating file system. MFT Multi-Programming Fixed Tasks, had 15 fixed size partitions defined at boot time. MVT Multi-Programming Variable Tasks, had up to 15 partitions defined dynamically. RTOS Real Time Operating System, run on 5 NASA custom System/360/75s. A mash up by the Federal Systems Division of the MFT system management, PCP basic kernel and file system, with MVT task management and FSD custom real time kernel extensions and error management. The pinnacle of OS/360 development. OS/370 The official port of OS/360 targeted for the System/370 virtual memory architecture. Customer installations in the following variations: OS/VS1 Virtual-memory version of OS/MFT OS/VS2 Virtual-memory version of OS/MVT SVS Single Virtual Storage (both VS1 & VS2 began as SVS systems) MVS Multiple Virtual Storage (eliminated any need for VS1) OS/390 Upgrade to MVS, with an additional Unix-like environment. z/OS z/Architecture version of OS/390. TPF z/OS extension CP/CMS Control Program / Cambridge Monitor System, Virtual Machine operating System for System/360 Model 44 and 67 VM/CMS Virtual Machine / Conversational Monitor System, VM (operating system) for System/370 with Virtual Memory. VM/XA VM (operating system) eXtended Architecture for System/370 with extended Virtual Memory. VM/ESA Virtual Machine /Extended System Architecture, added 32 bit addressing to VM series. z/VM z/Architecture version of the VM OS (64 bit addressing). IBM System/34, 36 System Support Program, or SSP OS/400 descendant of System/38 CPF i5/OS extends OS/400 with significant interoperability features. AIX (a System V Unix version) AOS (a BSD Unix version) Linux (IBM has contributed much code to this open source operating system, listed below) PC-DOS IBM supported, documented, and licensed copies of Microsoft MS-DOS OS/2 (developed jointly with Microsoft) OS/2 Warp eComStation (Warp 4.5/Workspace on Demand, rebundled by Serenity Systems International) IBM 8100 DPCX IBM 8100 DPPX K42 PowerPC or Intel x86 based cache-coherent multiprocessor systems (IBM Website) IBM EDX Event Driven Executive for the IBM/Series 1 minicomputers IBM RPS Realtime Programming System for the IBM/Series 1 minicomputers MicroC/OS-II (Small pre-emptive priority based multi-tasking kernel) Xenix (licensed version of Unix; sold to SCO in '90s) MSX-DOS (developed by MS Japan for the MSX 8-bit computer) MS-DOS (developed jointly with IBM, versions 1.0–6.22) Windows CE (OS for handhelds, embedded devices, and real-time applications that is similar to other versions of Windows) Windows CE 3.0 Windows Mobile (based on Windows CE, but for a smaller form factor) Windows CE 5.0 DOS based Windows Windows 1.0 Windows 2.0 Windows 3.0 (the first version to make substantial commercial impact) Windows 3.1x Windows 3.2 (Chinese-only release) Windows 95 (aka Windows 4.0) Windows 98 (aka Windows 4.1) Windows Millennium Edition (often shortened to Windows Me) (aka Windows 4.9) OS/2 (developed jointly with IBM) Windows NT Windows NT 3.1 Windows NT 3.5 Windows NT 3.51 Windows NT 4.0 Windows 2000 (aka Windows NT 5.0) Windows XP (aka Windows NT 5.1) (codename: Whistler) Windows Server 2003 (aka Windows NT 5.2) (codename: Whistler Server) Windows Fundamentals for Legacy PCs (aka Windows NT 5.1) Windows Vista (aka Windows NT 6.0) (codename: Longhorn) Windows Server 2008 (aka Windows NT 6.0) (codename: Longhorn Server) Windows 7 (previously codenamed Blackcomb, then Vienna) Windows Preinstallation Environment (WinPE) NetWare network operating system providing high-performance network services. Has been superseded by Open Enterprise Server line, which can be based on NetWare or Linux to provide the same set of services. SUSE Linux acquired by Novell which has adopted it as its core infrastructure. Novell now is a prime contributor to open-source projects based on Linux. LISP machine Operating Systems ran on specialized processors with LISP implemented as microcode Symbolics Genera written in a systems dialect of the Lisp programming language called ZetaLisp. Genera was ported to a virtual machine for the DEC Alpha line of computers. Texas Instruments' Explorer Lisp machine workstations also had systems code written in Lisp Machine Lisp. The Xerox 1100 series of Lisp machines ran an operating system that was also ported to virtual machine called "Medley." Lisp Machines, Inc. also known as LMI, also ran an operating system based on MIT's ZetaLisp. The Mesa programming language was used to implement the Pilot operating system, used in Xerox Star workstations. PERQ Operating System (POS) was written in PERQ Pascal. EOS (Operating System), developed by ETA Systems for use in their ETA-10 line of supercomputers EMBOS, developed by Elxsi for use on their mini-supercomputers GCOS is a proprietary Operating System originally developed by General Electric PC-MOS/386 - DOS-like, but multiuser/multitasking SINTRAN III - an operating system used with Norsk Data computers. THEOS TinyOS TRS-DOS A floppy-disk-oriented OS supplied by Tandy/Radio Shack for their Z80-based line of personal computers. TX990/TXDS, DX10 and DNOS - proprietary operating systems for TI-990 minicomputers MAI Basic Four - An OS implementing Business Basic from MAI Systems. Michigan Terminal System - Developed by a group of American universities for IBM 360 series mainframes MUSIC/SP (an operating system developed for the S/370, running normally under VM) TSX-32, a 32-bit operating system for x86 platform. OS ES An operating system for ES EVM Prolog-Dispatcher - used to control Soviet Buran space ship. Aegis (Apollo Computer) Amiga Unix (Amiga ports of Unix System V release 3.2 with Amiga A2500UX and SVR4 with Amiga A3000UX. Started in 1989, last version was in 1992) Clix (Intergraph's System V implementation) Coherent (Unix-like OS from Mark Williams Co. for PC class computers) DNIX from DIAB DSPnano RTOS (POSIX nanokernel, DSP Optimized, Open Source) Idris workalike from Whitesmiths INTERACTIVE UNIX (a port of the UNIX System V operating system for Intel x86 by INTERACTIVE Systems Corporation) IRIX from SGI MeikOS NeXTSTEP (developed by NeXT; a Unix-based OS based on the Mach microkernel) OS-9 Unix-like RTOS. (OS from Microware for Motorola 6809 based microcomputers) OSF/1 (developed into a commercial offering by Digital Equipment Corporation) OPENSTEP QNX (POSIX, microkernel OS; usually a real time embedded OS) Rhapsody (an early form of Mac OS X) RISC/os (a port by MIPS of 4.3BSD to the MIPS RISC architecture) RMX SCO UNIX (from SCO, bought by Caldera who re-renamed themselves SCO Group) SINIX (a port by SNI of Unix to the MIPS RISC architecture) Solaris (Sun's System V-based replacement for SunOS) SunOS (BSD-based Unix system used on early Sun hardware) SUPER-UX (a port of System V Release 4.2MP with features adopted from BSD and Linux for NEC SX architecture supercomputers) System V (a release of AT&T Unix, 'SVR4' was the 4th minor release) System V/AT, 386 (The first version of AT&T System V UNIX on the IBM 286 and 386 PCs, ported and sold by Microport) Trusted Solaris (Solaris with kernel and other enhancements to support multilevel security) UniFlex (Unix emulating OS by TSC for DMA-capable, extended addresses, Motorola 6809 based computers; eg SWTPC, GIMIX, ...) Unicos (the version of Unix designed for Cray Supercomputers, mainly geared to vector calculations) Unison RTOS (Multicore RTOS with DSP Optimization) DG/UX (Data General Corp) Project Epiphany CP Control Program. SDS later acquired by Xerox, then Honeywell. TRON (an open real-time operating system kernel) EXEC I EXEC II EXEC 8 Ran on 1100 series. Minix (study OS developed by Andrew S. Tanenbaum in the Netherlands) Plan 9 (distributed OS developed at Bell Labs, based on original Unix design principles yet functionally different and going much further) Inferno (distributed OS derived from Plan 9, originally from Bell Labs) Plan B (distributed OS derived from Plan 9 and Off++ microkernel) Solaris, contains original Unix (SVR4) code (code now open source via OpenSolaris project) Unix (OS developed at Bell Labs ca 1970 initially by Ken Thompson) Xinu, (Study OS developed by Douglas E. Comer in the USA) FreeBSD (one of the outgrowths of UC Regents' abandonment of CSRG's 'BSD Unix') NetBSD (one of the outgrowths of UC Regents' abandonment of CSRG's 'BSD Unix') OpenBSD forked from NetBSD GNU (also known as GNU/Hurd) GNU/Linux (commonly known as Linux) OpenDarwin OpenSolaris, contains original Unix (SVR4) code SSS-PC Developed at Tokyo University Syllable VSTa UnixLite Amoeba (research OS by Andrew S. Tanenbaum) Croquet House Haskell User's Operating System and Environment, research OS written in Haskell and C. ILIOS Research OS designed for routing

@borderlinux - nice list ;)