Other FOSS OS'

Open BSD

The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
OpenBSD team efforts emphasize portability, standardization, correctness, proactive security and integrated cryptography.

OpenBSD is developed entirely by volunteers.
The project's development environment and developer events are funded through contributions collected by The OpenBSD Foundation.
Contributions ensure that OpenBSD will remain a vibrant and free operating system

Free BSD

FreeBSD is an operating system for a variety of platforms which focuses on features, speed, and stability.
It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley. It is developed and maintained by a large community.

FreeBSD offers advanced networking, performance, security and compatibility features today which are still missing in other operating systems, even some of the best commercial ones.
It makes an ideal Internet or Intranet server.
Provides robust network services under the heaviest loads and uses memory efficiently to maintain good response times for thousands of simultaneous user processes.
With over 33,000 ported libraries and applications, FreeBSD supports applications for desktop, server, appliance, and embedded environments.

Net BSD

NetBSD is an entirely free and open-source UNIX-like operating system developed by an international community.
NetBSD was released in 1993. Over time, its code has found its way into many surprising environments.
As a community, the people who make NetBSD have a wide area of interests, which has resulted in a system with some diverse features:
Security and memory hardening features Powerful package management, Modern storage capabilities, ARM hardware support, Virtualization support...

Haiku

HAIKU is an open source operating system currently in development. Specifically targeting personal computing, Haiku is a fast, efficient,
simple to use, easy to learn, and yet very powerful system for computer users of all levels.

Additionally, Haiku offers something over other open source platforms which is quite unique:
The project consists of a single team writing everything from the kernel, drivers, userland services, tool kit,
and graphics stack to the included desktop applications and preflets.

ArcaOS

Do you have a custom written OS/2 application? Favorite, classic OS/2 productivity, word processing, spreadsheet, or graphics applications? OS/2, Windows 3.1, or DOS games? ArcaOS runs all of those just like OS/2 Warp 4, only better.ArcaOS is more compatible with modern hardware, makes more efficient use of memory and system resources, and installs more easily than any other OS/2 distribution…ever.Really.

Do you have a system with 16GB of RAM in it? Want your apps to really fly? Configure ArcaOS to utilize all memory above 4GB as a RAM disk, and at bootup, copy your most frequently used applications there. It’s like running your OS/2, Windows, DOS, REXX, Java, and ported Linux applications on air.

ReactOS

ReactOS , Windows all run all software and drivers that run the operating system, a free Windows operating system and aims to have the option of open source and free an operating system.

ReactOS, through reverse engineering methods andMSDNIt was written using various sources such as sources. Installing ReactOS is like installing Windows, and ReactOS can run most Windows software properly. ReactOS can be downloaded as CD, live CD, VirtualBox , QEMU, and VMware templates installed from their website. There is no Turkish language support in ReactOS.

FreeDOS

FreeDOS is an open source developed DOS-based operating system under the GNU General Public License , born to keep free DOS alive after Microsoft stopped developing MS-DOS in 1999.
Its aim is to be 100% compatible with DOS and this goal has been almost completely achieved.
With this operating system, it is possible to connect to the Internet with very old computers using the open source Arachne web browser.
Computers that are sold without any operating system installed in the market are also offered as FreeDos.
Some users prefer desktop computers and laptops with FreeDos installed at a cheaper price due to the fact that FreeDos is free.

Features:

It's free.
It is 100% open source. (Distributed under the GNU License).
It is based on DOS.
It can run under DOSBOX or DOSEMU ( dos emulator under GNU / Linux ).
It can run old DOS applications without any problems.
It has long filenames and FAT32 support.
It has USB support.
It has a 32-bit version.

KolibriOS

KolibriOS is written in the FASM assembly language and based on the source code of the MenuetOS operating system.
It boots in a flash and gives you access to a number of useful apps.
There’s no installation involved, though you’ll have to make sure you select the option to save the changes you made during the session when you shut it down.

The only shortcoming of the OS is that its productivity apps aren’t really mature enough for everyday use, and the lack of a package manager doesn’t help its case either. On the other hand, if you like retro gaming there’s no better OS.

Syllable Desktop

Developed between 1994 and 2001, AtheOS – initially planned as a clone of AmigaOS but later following its own path – was the work of a lone Norwegian programmer, Kurt Skauen. After Skauen abandoned the project, its GPL-licensed source code was picked up by the community and Syllable Desktop was born.
The majority of it is composed of unique code, although certain components have been pulled from the vast library of open source Linux programs; there's also a Server version, which is more traditionally Linux.

Syllable's key selling point – ignoring the fact that it's free – is its speed and lightness. The creators recommend a Pentium CPU with 32MB of RAM, which should give you an idea of how lightweight it is. Slap this on a modern PC and you'll likely never have seen an OS so quick. Syllable does lack the ports that make other indie operating systems attractive, though it contains a number of native apps for web browsing, email, VNC and more. We're not entirely positive that it's still active – the last official update was some time in 2012 – but if there's a very, very old PC you need to resurrect with reasonably modern system architecture, try this.

SkyOS

The development of SkyOS has sadly been halted, but it's still worth looking at as an example of an OS constructed from scratch.
Developed initially as an open source project by coder Robert Szeleney, SkyOS was based on concepts gleaned from other platforms but didn't originally borrow their code.
That said, a few components are based on other packages – there's no sense, for example, developing an entirely new compiler when GCC already exists, and the SkyFS filesystem is forked from OpenBFS. Later in its life, Szeleney appears to have experimented with a version of SkyOS built on top of a Linux kernel in an attempt to help with driver compatibility.

The source was closed midway through its life, and Szeleney continued development based on feedback from a popular (paid) public beta program. Unfortunately the struggle to keep up with ever-diversifying computing standards became too much for the lone coder, and development was halted in 2009, with the most recent beta made publicly available in 2013. It's obviously incomplete, and not suitable for any kind of business environment, but as a curio to run within a VM it's very interesting.

TempleOS

Whether the extreme religious doctrine behind it interests you or not, TempleOS is an interesting example of a completely independent, unique OS. It's been made and maintained with extreme dedication by one man, Terry A. Davis, over the course of ten years.

TempleOS – programmed entirely using Davis' own language, the excellently named HolyC, which you also use to interact with its shell – deliberately includes no networking and absolutely no hardware support beyond that which forms the core PC system.
So what's the point?
TempleOS has been built from the ground up with what seems like no hang-ups on existing operating systems. The entire thing is hyperlinked, meaning you can quickly burrow down to the source of a program just as easily as you can find its dependencies, and it's super-quick; there's no paging, so the whole OS gets up and running within a second or two.