Haiku is described as 'open-source operating system that specifically targets personal computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn and yet very powerful' and is a popular app in the OS & Utilities category. There are more than 100 alternatives to Haiku for a variety of platforms, including Linux, Self-Hosted solutions, BSD, Windows and Mac. The best alternative is Debian, which is both free and Open Source. Other great apps like Haiku are Linux Mint (Free, Open Source), Manjaro Linux (Free, Open Source), Arch Linux (Free, Open Source) and Fedora (Free, Open Source).
MerOS is a user friendly, good looking, general purpose Nix based GNU/Linux distro. It is ideal for those who want the most out of their computers, a well-designed desktop environment and an uncommon Linux experience.
Obarun is a GNU/Linux operating system that relies on s6/66 init and service management suite. The goal of Obarun is to provide an alternative for people looking for more simplicity and transparency in maintaining their systems.
The CLIP OS project is an open source project maintained by the ANSSI (National Cybersecurity Agency of France) that aims to build a secure, multi-level operating system, based on the Linux kernel and a lot of free and open source software.