OpenBSD

OpenBSD is a free and open source Unix-like operating system that is generally focused on security. OpenBSD was originally a fork of NetBSD, but has since become it's own project. OpenBSD is responsible for many open source projects that are widely used today, including OpenSSH, LibreSSL, OpenSMTPD, spamd and PF. The macOS kernel, Darwin, borrows many utilities and libraries from OpenBSD.