From the most consumer focused distros like Ubuntu, Fedora, Mint or elementary OS to the more obscure, minimal and enterprise focused ones such as Slackware, Arch Linux or RHEL, I thought I've seen them all. Couldn't have been any further from the truth. Linux eco-system is very diverse. There's one for everyone. Let's discuss the weird and wacky world of niche Linux distros that represents the true diversity of open platforms.
Puppy Linux: An operating system which is about 1/10th the size of an average DVD quality movie rip, that's Puppy Linux for you. The OS is just 100 MB in size! And it can run from RAM making it unusually fast even in older PCs. You can even remove the boot medium after the operating system has started! Can it get any better than that? System requirements are bare minimum, most hardware are automatically detected, and it comes loaded with software catering to your basic needs. Experience Puppy Linux.
Suicide Linux: Did the name scare you? Well it should. 'Any time - any time - you type any remotely incorrect command, the interpreter creatively resolves it into rm -rf / and wipes your hard drive'. Simple as that. I really want to know the ones who are confident enough to risk their production machines with Suicide Linux. Warning: DO NOT try this on production machines! The whole thing is available in a neat DEB package if you're interested.
Qubes OS: Qubes is an open-source operating system designed to provide strong security using a Security by Compartmentalization approach. The assumption is that there can be no perfect, bug-free desktop environment. And by implementing a 'Security by Isolation' approach, Qubes Linux intends to remedy that. Qubes is based on Xen, the X Window System, and Linux, and can run most Linux applications and supports most Linux drivers. Qubes was selected as a finalist of Access Innovation Prize 2014 for Endpoint Security Solution.
Ubuntu Satanic Edition: Ubuntu SE is a Linux distribution based on Ubuntu. "It brings together the best of free software and free metal music" in one comprehensive package consisting of themes, wallpapers, and even some heavy-metal music sourced from talented new artists. Though the project doesn't look actively developed anymore, Ubuntu Satanic Edition is strange in every sense of that word. Ubuntu SE (Slightly NSFW).
Tiny Core Linux: Puppy Linux not small enough? Try this. Tiny Core Linux is a 12 MB graphical Linux desktop! Yep, you read it right. One major caveat: It is not a complete desktop nor is all hardware completely supported. It represents only the core needed to boot into a very minimal X desktop typically with wired internet access. There is even a version without the GUI called Micro Core Linux which is just 9MB in size. Tiny Core Linux folks.
NixOS: A very experienced-user focused Linux distribution with a unique approach to package and configuration management. In other distributions, actions such as upgrades can be dangerous. Upgrading a package can cause other packages to break, upgrading an entire system is much less reliable than reinstalling from scratch. And top of all that you can't safely test what the results of a configuration change will be, there's no "Undo" so to speak. In NixOS, the entire operating system is built by the Nix package manager from a description in a purely functional build language. This means that building a new configuration cannot overwrite previous configurations. Most of the other features follow this pattern. Nix stores all packages in isolation from each other. More about NixOS.
GoboLinux: This is another very unique Linux distro. What makes GoboLinux so different from the rest is its unique re-arrangement of the filesystem. It has its own subdirectory tree, where all of its files and programs are stored. GoboLinux does not have a package database because the filesystem is its database. In some ways, this sort of arrangement is similar to that seen in OS X. Get GoboLinux.
Hannah Montana Linux: Here is a Linux distro based on Kubuntu with a Hannah Montana themed boot screen, KDM, icon set, ksplash, plasma, color scheme, and wallpapers (I'm so sorry). Link. Project not active anymore.
RLSD Linux: An extremely minimalistic, small, lightweight and security-hardened, text-based operating system built on Linux. "It's a unique distribution that provides a selection of console applications and home-grown security features which might appeal to hackers," developers claim. RLSD Linux.
Did we miss anything even stranger? Let us know.