This is another BSD 4.4 based OS. It is much better in interface than FreeBSD and OpenBSD but is least used BSD. Like all modern open source BSD it is compatible with linux using binary compatibility layer. Its use is more extensive from embedded systems to entreprise servers.

OpenBSD forks from NetBSD. Unlike former it supports all hardware. Installation too is not damn difficult for me. Installation is through ports which is a little same as portage in gentoo. It is I think second most secure OS in the world ( about OpenBSD no one has any doubt). Its bootmanager is Xen compatible and probably it gives the best performance for Xen in all Oses.
Its file systems is ZFS same as solaris. It nearly supports all of the Linux and Mac OS file systems. Even then its adoption is pretty limited. Generally in embedded systems and data centers having network stacks. It can be used as desktop OS also.