I'm currently speaking to you through a virtual machine running HaikuOS!
What is HaikuOS?
It's a free and open-source revival of BeOS, which was a unix-based operating system from the mid-90s, made by ex-Apple developers. I heard about it first through Action Retro, who made a few videos on Haiku.
Pros:
Free & Open Source
Frequently gets updated and supported
Lightning fast! Seriously, I'm running this on a virtual machine that isn't particularly powerful (4-core CPU, 16GB RAM, 20GB Hard Disk) and it boots almost instantly, way faster than Windows or even Linux Mint (my main OS of choice) does on real hardware.
Funky 90's aesthetics
Super fine control over your computer--especially if you have a "if you can't explain what you're used for, you're getting uninstalled" attitude towards apps.
Comes with a bunch of built-in utilities, as well as a handful of demonstrations, and a full Haiku OS guide.
Has a pre-install live environment for testing before installation.
Fully-featured GUI application manager, complete with staples like Libreoffice, Inkscape, Krita (and various other KDE apps), etc.
Cons:
Not Linux; can't run Linux applications and is not based on the Linux kernel, so you'd need to rebuild any application you'd like to run on Haiku
Default web browser (WebPositive) sucks really bad for anything beyond simple HTML web browsing. Fortunately, Falkon (a chromium-based browser made by KDE) is available on Haiku's software manager, HaikuDepot, and that's what I'm using to write this post.
Layout really takes some getting used to. That's not really an issue if you're a particularly fast learner, or just don't like either Windows or MacOS' layouts.
Ctrl and Alt (on Windows/Linux keyboards) are swapped for most keyboard shortcuts, which is very jarring if you're not used to it. Not a huge deal, and it is supposedly reversible (although I haven't found the setting for it.)
Now, what do I think of Haiku OS? Uh... well...
For me, an OS needs to have wider compatibility and a better UX than what Haiku has to offer. You can't, for example, download Discord or Steam, much less Steam games. That isn't Haiku's fault, necessarily, but it is something to keep in mind. I want to also make clear that this is also why I don't like using MacOS; aside from just disliking Apple, it just isn't really meant for games and is designed primarily for an office environment, making it unideal for my purposes.
If you choose to use Haiku, it seems to be primarily an "Office" operating system, with a focus on productivity rather than gaming. That's perfectly fine--in fact, I would argue that having an OS where gaming isn't an option may be a very good thing for productivity--but for my PCs, it's not something I would choose.
That said, I do 100% support their mission and I intend to donate once my wallet gets on the same page. While I like Linux Mint the most out of any operating system I've tried thus far, I think that there should be more OSes out there that aren't based on the mainstream kernels, even if I don't personally like them; competition is a good thing!
If you're interested, here's a link to their website, where you can find download links & more information: https://www.haiku-os.org/
I seriously recommend donating to their cause or otherwise contributing what you can. Alternative OSes need more attention from the wider public!