You want to try something interesting but want to dual-boot. That last bit could be difficult or "impossible" but using a VM or running from USB stick are options.

• https://www.haiku-os.org I've run it from USB stick on some older laptop.
• https://chimera-linux.org FreeBSD user-land with a Linux kernel.
• https://nomadbsd.org FreeBSD which can be run from USB stick with persistent storage. Has a version with ZFS support.
• https://nixos.org Very interesting concept.
• https://www.gobolinux.org GoboLinux is an alternative Linux distribution which redefines the entire filesystem hierarchy. Doesn't seem up to date but quite interesting. If I remember well you can have different versions of software installed at the same time. Let's say (making this up) Bash 1.1, 3.1 and 5.2
• https://bedrocklinux.org Bedrock Linux is a meta Linux distribution which allows users to mix-and-match components from other, typically incompatible distributions.

Fedora silverblue
The main system OS is immutable and tracked by a git like system, which means to upgrade, or downgrade your whole OS to a release you just pull in the 'tag' you want, and it just does it.

Can also side grade easily to respins of the OS using this too, just add the remote and pull in the image.

It's really hard to go wrong with Debian.

That's my safe answer.

If you share more about your interests, hobbies, I might have other ideas.

I suppose, when in doubt, there's always Linux From Scratch. It's a very interesting experience.

Perhaps TempleOS and unholy forks of it?

GNU Guix

If you're looking for interesting more than useful, may I present TempleOS.

Redox OS or React OS lol

Qubes - an OS that compartmentalizes system functions (including userspace) into separate VMs, with the intent of keeping them secure from each other. Kind of an internal zero-trust approach. Complicated to use.

Alpine Linux - stripped down to create a reduced attack surface, with the intent to provide only packages which have been vetted for security. Fairly straightforward.

Redox OS - a Unix-like OS written in Rust (not actually Linux). Limited, still kind of a prototype.

Damn Small Linux has been revived with a new version recently, which is nice to see.

HoloISO - a community built reimplementation of the Steam Deck OS.

AROS (Amiga) Research Operating System

Wikipedia

Official Page

would run in an emulator or bare metal boot from separate media.

For something interesting, I suggest Qubes OS.

For a reliable workhorse, I would suggest Debian.

I was gonna slap Kali on there for fun, but that's a little obvious.

ReactOS is kinda piquing my interest.

opensuse aeon (Linux). Immutable and designed to just work and keep working. https://en.opensuse.org/Portal:Aeon

NetBSD

something interesting you say. source mage linux. https://sourcemage.org/ its more up to date than it seems. look at the repo.

For something really interesting, try GhostBSD, or one of the other BSDs (free, open, net)

Updates: We are currently running Fedora SilverBlue, which is a very pretty OS as they go.

We won't stop with that, though, so keep the suggestions coming!

I haven't had an opportunity to test it myself yet, but I've heard very good things about NixOS. The basic premise is that all of your system state, every config file in /etc, every package you have installed, everything, is defined by a single configuration file called configuration.nix. If you back up just your home directory and that file, you can plonk it into a brand new copy of NixOS, run a single command, and have it redownload and re-set-up everything else exactly how you left it.

Sugar, from One Laptop Per Child. it has like an entirely different UI paradigm or something, haven't tried it

freedoss 9front Uxn(more an emulator but still interesting to check out its by 100 rabbits) openbsd netbsd dragonflybsd etc...

Void, stable, fast, not a fork, also is using runit