BSD Family. It's a daemon, not a devil!

FreeBSD is an opensource Unix based operating system with an incredibly powerful networking stack. FreeBSD aims to be a general purpose server operating system and includes key features like IPFW, and ZFS, everything BTRFS wants to be.

OpenBSD is an opensource Unix based operating system with security being their main focal point. OpenBSD originally introduced Packet Filter (PF), a feature rich firewall engine, and OpenSSH.

DragonflyBSD is based from FreeBSD but has spun off to focus on being highly scalable. DragonflyBSD also created the HAMMER filesystem, an advanced filesystem supporting snapshots and is never inconsistent.

NetBSD aims to be a clean portable operating system that can be run on everything, including toasters.