If you are not using macOS or Windows, in all likelihood, your operating system is one of the Linux distributions. Of course, for many people who have never had any interest in computer science or programming, Linux has still been a mystery despite the fact of its growing popularity among less experienced users. Yet, there is a variety of free operating systems unrelated to Linux which you might be willing to use.
Note, some of these systems can actually be used instead of your current operating system whereas others are rather experimental. Anyway, trying one of them is a great fun!
The most popular free operating systems which are not based on Linux are the distributions if BSD. There is a variety of such systems including PC-BSD, OpenBSD, NetBSD and FreeBSD. The latter one is especially popular due to the fact it goes with a highly permissive licence. As a result, many other operating systems and devices are using parts of its code. It is quite possible that you have one of them right now and are not even aware of it. For example, Juniper routers, the Sony PlayStation 4 and even Apple macOS are using the parts of the code of FreeBSD.
This operating system is probably one of the most reliable and practical in case you want to use something else rather than Linux. In addition to it, you will find a lot of amazing software running on this operating system.
FreeDOS is an option for people who still remember MS-DOS and are willing to feel the taste of using this operating system once again. Of course, it is a crucial component required for running games designed for MS-DOS in the first place.
The difference between FreeDOS and classic MS-DOS is certainly the fact FreeDOS will provide you with MS-DOS experience on modern software. It can also be used inside a virtual machine.
You might have already heard about an operating system called Solaris which was first developed as a closed-source project however, was changed into an open-source project in 2008. In such a way, the operating system was transformed into OpenSolaris. This was the product of Oracle which discontinued OpenSolaris in 2010. Then, Oracle came back to the proprietary form of an operating system developing Solaris 11.
Yet, OpenSolaris had managed to gain attention of people who liked it to the point that they made an attempt to keep the system available even though Oracle was not working on this project anymore. In such a way, illumos was created. Note that this operating system is a part of several distributions which are used instead of directly using illumos. The most notable examples of these distributions are openindiana and DiLOS.
If you have been passionate about operating systems and computers for quite a while, you might be aware of a graphical operating system created by Be Inc specifically for the BeBox devices. This programme was used back to 1995.
Even though BeOS had never been particularly popular, it had managed to collect a group of fans who still want to see this operating system running. In such a way, they created the experimental project called Haiku, the aim of which is to be compatible with the programmes created for BeOS. If you are interested in using the programmes runnable on BeBox, you should try installing Haiku.