ChromeOS Flex
ChromeOS Flex, developed by Google, is a free and lightweight Linux distribution based on Gentoo-derived ChromeOS. Unlike ChromeOS which is designed specifically for Chromebook computers, ChromeOS Flex can be installed on most x86_64 devices with a AMD or Intel processors, offering a Chromebook-like experience.
SmartOS
SmartOS is an open-source UNIX-like operating system based on illumos, a community fork of OpenSolaris. It features four technologies - ZFS (a combined file system and logical volume manager), DTrace (a dynamic tracing framework for troubleshooting kernel and application problems), Zones (a lightweight virtualisation solution), KVM and bhyve (two full virtualisation solutions for running a variety of guest operating systems, including Linux, Windows, BSD and Plan9).