Do you ever get the urge to try random obscure operating systems? I think it's sometimes just fun to use systems that are not widely used. What obscure operating systems have you tried (or have thought about trying)? I've been looking into Haiku  lately.
ReactOS® is a free, modern operating system based on the design of Windows® XP/2003. Written completely from scratch, it aims to follow the Windows® architecture designed by Microsoft from the hardware level right through to the application level. This is not a Linux based system, and shares none of the unix architecture.
The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows. This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, such that people accustomed to the familiar user interface of Windows® would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to remove Windows® and install ReactOS without the end user noticing the change.
MenuetOS is an Operating System in development for the PC written entirely in 32/64 bit assembly language, and released under the License . It supports 32/64 bit x86 assembly programming for smaller, faster and less resource hungry applications.
Menuet has no roots within UNIX or the POSIX standards, nor is it based on any operating system. The design goal has been to remove the extra layers between different parts of an OS, which normally complicate programming and create bugs.
Menuet's application structure is not specifically reserved for asm programming since the header can be produced with practically any other language. However, the overall application programming design is intended for easy 32/64 bit asm programming. Menuet's responsive GUI is easy to handle with assembly language.
BeOS  http://en.wikipedia.org/wiki/BeOS
Windows 3.1 or earlier.
I wouldn't normaly suggest any windows product for something like this. But, there is enough people now that have never seen nor touched windows 3.1 or earlier. It is a good wakeup call on how far we have come in the windows world.
And if you are really cool. Windows 3.11 for Workgroups
Haiku , a BeOS clone, is quite nice, and resembles Unix a little. http://haiku-os.org/
OS/2  - Great OS for its time. I believe OS/2 is destined to be the most important operating system, and possibly program, of all time. As the successor to DOS, which has over 10,000,000 systems in use, it creates incredible opportunities for everyone involved with PCs. -- Bill Gates http://en.wikipedia.org/wiki/OS/2
Geos  (GeoWorks). Used by the very first versions of America Online when they changed to a graphical interface. It was a full GUI that fit on a single 3.5" floppy with room for both the OS and an application. http://en.wikipedia.org/wiki/GeoWorks
I've fiddled around a lot with Plan 9 
Great OS, made by the same people that originally made Unix. It's and a research project and not that modern anymore. It's taken the unix philosophy to the next level and it's a bit sad unix didn't adopt these philosopies , aldthough a few have crept in. http://plan9.bell-labs.com/plan9/
Apple IIgs System 6. Impressive for its time.
NeXTSTEP  - it only ran only thier proprietary computers for the most part, but it's cool to see where Mac OS X got a lot of its ideas/concepts from. http://en.wikipedia.org/wiki/Nextstep
Not everyone would consider it obscure, but OpenVMS  is certainly less common (In the desktop space anyway). http://www.openvmshobbyist.com/
MikeOS  - a nice, educational OS written entirely in 16-bit real mode assembly language! http://mikeos.berlios.de
How about Andrew Tanenbaum's Minix ? For anyone starting with OS development, this is a must, a classic. I'm surprised nobody mentioned it yet. http://en.wikipedia.org/wiki/MINIX
There are a few rather graphically impressive obscure operating systems. You may have trouble actually trying the latter two. I particularly wish I had the hardware to mess around with MorphOS.
NeXTSTEP shipped for 5 different arch (NeXT hardware (68k), SPARC, PARISC, x86 (damn, forget 5), and was in some stage of development for at least 3 (88k, Hobit, Alpha) more, an you could run app in NT (was that number 5?) and I think Solaris. It is the direct ancestor of OS X.
OS/2 was the thing, 15 years ago.
Beos was nifty, but missed it's shot. It took all of 7 seconds to go from finishing hardware boot on a dual Pentium Pro to ready for you to log into the GUI.
EROS seemed nifty, never tried it, http://en.wikipedia.org/wiki/EROS_%28microkernel%29
Booted Oberon once or twice. Nicolas Wirth's OS for 68k mac hardware.
I will try DexOS , when I get a chance. Looks original :)
From the wikipedia article :
DexOS, is a free and open source 32-bit games console type operating system for 32-bit x86 computers. It was written entirely in assembly language using FASM (flat assembler). The operating system's GUI was inspired by modern video game consoles but it also includes a command line interface. It was designed to boot from a 1.44 MB floppy disk and its kernel is less than 100KB. DexOS can also be booted from a CD, USB flash drive or hard drive.
N.A.C.H.O.S.  (Not Another Completely Heuristic Operating System)
A small, open source, educational OS used in a lot of Computer Science OS courses http://www.cs.washington.edu/homes/tom/nachos/
AROS  Research Operating System, AmigaOS clone for x86. It's based on Amiga OS 3.1 APIs. The A in AROS was originally Amiga. http://aros.sourceforge.net/
CP/M on my old Amstrad CPC664 (you used a boot disk), it was not unkown but certainly it was obscure.
QNX  was funny to play with at a time. It's a very small and light real-time OS mostly used for embedded systems, although it runs (ran?) on PCs.
They had a demo version booting from a floppy disk (1.44 Mb!), complete with TCP/IP, a graphical interface and a web browser. http://www.qnx.com/
I remember running (I think) Caldera OpenDOS as a replacement for MS-DOS. A DOS with mutli-tasking!
That was before we moved to Windows 3.1, then OS/2 Warp..
The OASIS ("Online Application System Interactive Software") Operating system written by Phase One Systems was a multitasking OS that ran on a 4mhz Z80 chip. RM-COBOL and BASIC and z80 assembler were the development tools
The original and best OS-9  by Microware. Not the Apple OS of the similar name. In the late '80's I was running OS-9 on a Tandy color computer with 512k of memory, on a 6809. This OS had genuine pre-emptive multi tasking. It is a great system for embedded work. http://www.microware.com/Products/Microware/OS-9-RTOS.html
VSTa . Microkernels were quite the thing in the '90s and VSTa was a research operating system that had a decent following but never quite achieved the following of other open-source unix-alikes. http://www.vsta.org:8080/
Oberon  is awesome. I've written about it here . It's a text-based graphical zoomable operating system, and in addition to booting with it, you can run it as an application inside other operating systems. http://en.wikipedia.org/wiki/Oberon%5Foperating%5Fsystem
Minix. Once tried installing Minix, Win 98 (back in 2000) and Linux on the same disk. Also, Syllable OS - thought am not sure how obscure it is now.
UNICOS  - a kind of Cray's Unix. http://en.wikipedia.org/wiki/Unicos
I will suggest DEC's TOPS-20  (aka TWENEX), popular from about 1975 to 1985. It reminds a bit about OpenVMS. The shell's user interface is interesting in particular, with immediate command help and guiding words.
An excellent emulator can be found here:
Have you ever heard of the Cassette Aided Operation System called CAOS? It was delivered with the KC 85  home computer built around a Z80 clone. http://en.wikipedia.org/wiki/KC_85
Xinu  is a nice educational OS developed at Purdue University.
I installed Red Hat 9 a few days ago on a VM.
By no means "obscure", but amazing how we traveled from that Linux version to the current one)
I sometimes power up IBM's MVS under the Hercules emulator. No real need, other than nostalgia for a misspent mainframe youth: