CentOS
CentOS as a group is a community of open source contributors and users which started in 2003 and has been sponsored by Red Hat since 2014. CentOS Linux versions up to CentOS Linux 8 are 100% compatible rebuilds of Red Hat Enterprise Linux, in full compliance with Red Hat's redistribution requirements.
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).