FreeBSD
FreeBSD is an open source Unix-like operating system that can be found (as a whole or in parts) in Apple macOS and iOS, Sony Playstation 4, Juniper routers and NetApp storage filers, to name a few. It also powers services such as Netflix and WhatsApp. I have been a developer with the project since 2003, and also contribute to related security-focused projects such as TrustedBSD and OpenBSM.