Linux

Maybe not some obscure ones, but here are some lesser known ones:

Talos Linux. It's an immutable operating system designed specifically to deploy kubernetes.

OpenSuse Harvester Think Proxmox, but instead of VM's and LXC containers, it's VM's and Kubernetes.

XCP-NG is a RHEL based distro designed for managing Linux virtual machines using the xen hypervisor, as opposed to KVM. Think Proxmox, but RHEL and Xen (also no LXC). However, it does not come with a web ui out of the box, you have to deploy it yourself. Technically, XCP is a Xen distribution, since Xen is a kernel with nothing but a hypervisor that runs under the main distro, but the primary management virtual machine is RHEL based, and uses Linux.

Speaking of Proxmox, Proxmox is technically a Linux distro.

SnowflakeOS is a project that aims to bring a GUI focused experience to NixOS.

TurnkeyLinux (site is loading very, very slowly for me right now) is not a single distribution, but rather a set of debian based distributions that are designed to be turnkey appliance virtual machines that contain and host a specific app. To deploy the app, all you have to do is set up the virtual machine.

Now, here are some not-linux, but interesting distros:

SmartOS. They ported KVM to unix, and also can use Linux syscall translation (similar to wine) to run apps in containers as well. There is also Bhyve. It's a very interesting hypervisor platform.

OmniOS is similar. Bhyve, KVM, and Linux syscall translation in containers.

Not really a Linux distro, but TempleOS

I see no one has mentioned Bedrock Linux yet. Not sure though how others would rate its 'obscurity' though. It's definitely a standout among distros.

There was this distro that stuffs everything of a package in one folder, instead of /usr/lib & co. What was it called again?

Kiss linux. Gobolinux. They are both alien, but interesting, each one in its own way.

If being usable is a metric, Slackware

The old PearOS(which looked like a meme-ish knockoff MacOS), UwUntu and Nyarch

postmarketOS. Way too underrated.

I imagine there was a time when this wasn't obscure, but I'm guessing people today don't remember Caldera OpenLinux. That was the first Linux distro I installed/used. A guy from church gave his copy.

Caldera eventually became SCO. But I'm pretty sure I was using Caldera OpenLinux before the whole Novell patent suit thing.

Obscure as in "only for a very specific purpose and nothing else"?...

Well, there is the Mircrosoft linux distro for their azure cloud

I guess DD-WRT as distro for router is also kind of obscure. Or the more general openWRT for embedded systems.

No one mentioned Bunsenlabs or Crunchbang Linux here, but they aren't really that obscure.

United Linux - the famous Red Hat Enterprise Linux killer!

https://en.wikipedia.org/wiki/United_Linux

2 days ago my friend found an old SATA hard drive and gave it to me to check what's on it, and me, not having a disk station or anything, and against all better judgment, I just swapped the disk in my laptop for my friend's, and instead of my laptop being fried it turned out the disk was running something called Crunchbang Linux

Jolicloud. I ran it on an old low-spec netbook in 2013ish, basically a ChromeOS before Chromebooks were a thing. It was discontinued in 2016 but great for the hardware while it lasted.

I haven't tried all that many distros, but I'd say Puppy Linux. Pretty neat that it loads into RAM from USB and has fairly light memory requirements, but it does feel a little on the clunky side as far as configuration and stuff goes.

Linux STD! Waaaay before skiddos had backtrack or kali

SLiTaz

It's an obscure originally live usage oriented distro that you could also install. It was the first *Nix I ever used.