An OS with an almost legendary focus on security and stability, OpenBSD is the software that took out its "Linux compatibility layer" because of concerns around the security of keeping it.
If you've not heard of or used OpenBSD, I can guarantee that you've at least used an associated project. Some that fall under the OpenBSD realm are the following:
- httpd (that's the OpenBSD-specific package, not re-badged Apache)
This means that while you've probably never SSH'd onto an OpenBSD system itself, you're still using software that's built primarily on OpenBSD systems. I know of no distribution of Linux that doesn't use OpenSSH as the default SSH client and server (though alternatives do exist).