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Many open source operating systems are available for users, Linux being only one of them.

FreeBSD is probably second only to Linux among open source operating systems in popularity and community size. It's a Unix-like (some say even more Unix-like than Linux) free operating system released under the BSD license and derives from the code of the 4.2 BSD Unix operating system. FreeBSD is today a reasonable desktop alternative to Windows or Linux, although its "user-friendliness" is comparable to that of a Debian or Slackware distribution, and is therefore not recommended for newbie users. Many of the command line tools are similar and it supports the same graphical and desktop interfaces of Linux, including KDE and Gnome.

Note that there is nothing like a FreeBSD distribution as is the case with Linux - the whole development is more centralized, and the word "FreeBSD" refers to the whole OS, not only to the kernel. The fact that the development model differs while there are three main flavors, makes this non-obvious to the casual inquirer, however - it seems like three 'distros' of *BSD. Also, there are some spin-offs that have been developed recently such as FreeSBIE, a live CD of FreeBSD in the fashion of Knoppix. DragonFly is another example and PicoBSD was a sort of 'BSD on a floppy' that has been around even longer. The Debian project is also beginning to work on a Debian distribution based on the FreeBSD kernel.

NetBSD is another BSD-derived Unix-like operating system. It is similar to FreeBSD but is focused on portability. In fact, NetBSD can run on a striking variety of hardware platforms, including the Playstation game console!

OpenBSD is a fork of NetBSD. Led by the idiosyncratic personality of a former NetBSD developer, Theo de Raadt (unlike the other *BSD systems, that don't have a single leader but a core team), OpenBSD is a BSD Unix-like system with a strong commitment to security and networking. OpenBSD releases try to be "secure by default", developing and using clever and advanced technologies like W^X and using ProPolice by default in its inclusion of gcc to successfully avoid common security problems. Although regarded as probably the least user-friendly and the most networking-oriented OS of the *BSD family, OpenBSD focuses also on providing useful and detailed documentation for use of the operating system. OpenBSD is usable as a desktop environment.

Hurd is the official kernel of the GNU project. It is another Unix-like system, but it has a very different technical design from both Linux and the *BSDs. Where Linux or *BSD are monolithic kernels, GNU Hurd is made up of a microkernel, implementing the GNU version of the Mach kernel (GNU Mach), that handles the most basic functions (memory management etc.), and a bunch of programs - collectively named the Hurd - that handle most functions that, in common operating systems, are performed by the kernel: filesystem management, networking capabilities, etc.

Despite the great technical elegance of the microkernel design, and the theoretical advantages of such a design for developers, GNU Hurd development was very slow. Work on the Hurd began even before the birth of Linux but, as of today, is still a very immature and unstable operating system. There is still no true installer (users must install Hurd from another Unix-like system, like Linux or FreeBSD), X Window System support is limited and almost always needs configuration-hacking by hand, and there are severe hardware support limitations (no shared IRQs support, no support for filesystems larger than 1-2 Gbyte). The GNU Hurd is considered a great OS for learning and hacking, but it is probably not advisable as a desktop or production system.

The Debian project releases the only one current GNU Hurd distribution. Debian GNU Hurd is a small bootable CD-ROM ISO that performs most of the installation steps (becoming essentially similar to a Debian GNU/Linux installation) and a lot of ready to use packages.

Outside of the Unix family, there are open source DOS clones such as FreeDOS, NT clones such as ReactOS, innumerable BeOS clones such as OpenBeOS, and more original operating systems such as Atheos/Syllable/et al, and Menuet. Few of these are yet usable for a dedicated sole desktop system, but several are installable and usable by people with only average computer skills.