Linux

Not super obscure, but not many talk about it. Q4OS, I love it, a perfect windows replacement down to even imitating the old style windows installer. Plus it's Debian based so it has a lot of support. I plan on moving my grandparents to it when windows 10 gets fully discontinued as their current rig doesn't support 11

Longene Linux. Linux-based operating system kernel intended to be binary compatible with application software and device drivers made for Microsoft Windows and Linux.

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

postmarketOS. Way too underrated.

Rebecca Black OS.
It is the only Linux distro to date built around Weston, using Wayland's full capability:

It doesn't include any Rebecca Black theming or is related to her in any way.
It's just called that cause the dev is a fan of hers.

Meego, a combination of Intel's Moblin and Nokia's Maemo. It only ever shipped on one device, the Nokia N9.

dyne:bolic - specifically 1.4.1

Had support for the original Xbox, a multimedia editing / streaming focussed OS. I'd never run it on mine - just messed with xdsl before going back to XBMC.

DSLinux

templeos /j

Bohdi linux. smoll and beautiful. Used to run it on my eeepc 701

Hannah Monata Linux and Red Star from North Korea.

Check out the random button on Distrowatch (distrowatch.com/random.php) - it's like a Linux lottery, but you always win something weird!

hyperbola

they have a wiki with insane nonsens about why they don't package certain things. Example:

pam
Package has different security-issues and is not oriented on the way of technical emancipation as Hyperbola is trying to adapt lightweight implementations.

https://wiki.hyperbola.info/doku.php?id=en:philosophy:incompatible_packages

Smoothwall. I used to run it a lot back in the early 2000s for personal use and even helped set up a couple small businesses with it but I don't hear of anyone else using it these days, people seem to love openwrt and pfsense more.

It was great for just taking any old x86 machine and making a powerful, fully featured firewall/router out of it, including a VPN server, all through a web interface. Nowadays that's boring shit but in 2002 it was pretty cool.

The first one that came to mind was fli4l (Floppy ISDN for Linux). Originally a distro of German origin that fit on a single floppy disk to turn a 386 or 486 PC into a router for ISDN connections. Last I looked it's still actively worked on.

There are probably tons of more obsuce ones. But this is one I actually used.

There was a bunch of weird rebadged Ubuntu derivatives back in the day.

Ubuntu satanic edition. https://archiveos.org/ubuntu-satanic/

Ubuntu Christian edition. https://archiveos.org/ubuntu-christian/

Hannah Montana Linux https://hannahmontana.sourceforge.net/

gobolinux

it's main feature is that it completely redefines the system's root directory structure. the only reason i even know it exists is because i'm friends with one of the creators

elive

you think a distribution that automatically includes all the proprietary stuff that we use baked into the distro would be more popular since it makes linux ready to go for most people; but it still gets fewer than 300 clicks per month.

Sabayon Linux. I'm not sure if it's still releasing updates, the main website is dead. It was based on Gentoo and later funtoo, but had a package manager of precompiled binaries. You could still use emerge if you wanted to. Definitely a weird and interesting distro

Blend OS is trying to do the declarative nixos thing but with an arch base. That's pretty cool.

ClearOS was Intel's attempt at an immutable os. From what I remember it was really fast.

Edit: actually it clear Linux not clearOS. Edit: also clear Linux is stateless. I don't know, there's a lot about it I don't understand

Red star OS

Have you ever heard of arch? That's what I use by the way

Limiting to those I have used daily and treated as Linux (used the terminal for example) probably Maemo. I used to carry my Nokia Internet Tablet 770, and then my N800 everywhere with me.

Maemo is also an ancestor of both Tizen and Sailfish OS

KISS

it's just a single bash script and a repository containing package definitions to compile them from source.

Basically LFS on drugs.

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

Jarro Negro. Made by Mexican students. And as far as I know, it's independent, not based on another distro.

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

Suicide linux. Nobody can run it for more than a day

Edit: i just searched "suicide linux" to see if it still exists and one of the top results was ian murdock's wiki page, :(