There are too many Linux distributions to list and too many Linux-based systems to try out just for fun. There also wouldn't be much of a point to it. And there's nothing to say about Windows or MacOS in this entry. But there is some mentioning of operating systems not very many people may have heard about.
The less big OSs
There are a few other operating systems that aren't used on as many computers, aren't as well-known as the big three and it's derivatives, but are still important for their use case or niche. There's the extended family BSD, MINIX, the PlayStation system and many other Unix-like systems, there's Illumos and other actual UNIXs, a variety of DOSs and many even less widely used OSs for small niches. Those all deserve an article. But this is not one about any of those.
The discontinued ones
There are many UNIXs and Unix-like operating systems that aren't actively developed or not even officially supported anymore but still in use. There are many old systems that are still sometimes used by retro computing fans, users with nostalgia attacks and people who just never updated their machines since the 80s or 90s. I'd like to write an entry about those systems. But not right now.
The unnoticed ones
There's also a huge landscape of operating systems on embedded systems that are not primarily seen as computers, such as DVRs, cars, household appliances, toys, parking meters and many many industrial tools and machines. An article about how this landscape of proprietary in-house systems turned into a stable of Linux systems would also be worth writing. Or one that takes a look at some sleek systems, from tiny, incredibly efficient environments for 90s microcontrollers to larger systems like Palm OS. But my motivation for this entry is about something else.
The interesting ones
And there are operating systems that don't fit any of the beforementioned categories, but still have a reason to exist or at least had a reason to be made at some point. Independently developed OSs for personal computers that emerged from a single person or a small group of computer enthusiasts as learning projects, to prove a point, or as a recreational coding project.
For a long time I thought that such operating systems don't exist, don't reach a state in which they are worth checking out or aren't shared publicly. I did wonder why nobody seems to write alternative OSs just for the sake of it. Maybe the landscape of existing OSs and the tiny adoption of any OS that isn't one of the big three (or four, or five, depending on where you make the cut) takes away any motivation to start yet another small OS from people who would otherwise pursue such an idea, I thought. But it turns out that I didn't look closely enough to find them. There totally are working alternative operating systems that were made for the sake of it, despite Linux satisfying all requirements of the project's initiators. Many of them may have 0 to 1 users and some of them may have never and may never be used as anybody's main OS. But some of them are very interesting systems. I'd like to point out a few of those.
My intention with this entry was to write a paragraph or two about one or two handful of OSs. But I found that there is more to write and more projects to share for different reasons. I will write separate entries about a dozen alternative operating systems when I find the time to check them out myself. I'll use the category Operating Systems for these entries. Since I found more than I thought I'll restrict myself to testing only systems that check the following boxes:
My impressions/conclusion so far
There are more alternative desktop operating systems than I expected. I thought after the 90s the landscape has become very flat. But development of a number of commercial and non-commercial operating systems went on for much longer. There have been many large projects I had never heard about. Some of those have been officially ended over the years (like eComStation and Syllable Desktop), others are still worked on (like Redox, MenuetOS and MorphOS). I stumbled over so many names of OSs that are in a state in which they're not really usable as a daily desktop OS, that had been published on some now offline website or that have been discontinued years ago. I thought I'd try out most of what I can find and write an overview of the alternative desktop OS landscape. But that would be a project in it's own that would be huge and take far more time than I want to invest in it. Many names worth mentioning have not been mentioned here and many cool projects also won't be mentioned in any of the articles to come. For example I ignored commercial OSs, like ArcaOS. I just don't want to pay 129 $ just to test a system that I'm likely not going to use.
I can already, after having only briefly tested a few systems, say that there are usable, stable systems that hardly anybody has heard of that are as good as any commercial OS from the 90s. That's good enough for most desktop users. Each with their own set of features and unique selling points, there are real decisions to be made when selecting an alternative operating system that go beyond the questions one asks when selecting the next Linux distribution during distro hopping. I am happy that there are more than one or two systems in a usable state that focus on supporting old hardware and/or using less hardware resources than we got used to with Windows, Linux and MacOS. Whether you're looking for something to run comfortably on an 80286 or something that runs ridiculessly fast on a modern 64 bit system. There are systems to choose from.