JunOS from Juniper is also based off of FreeBSD (I think they're moving to Linux, though) as (were?) NetApp filers (they made heavy use of the Berkley FFS snapshots back in the day).

FreeBSD was popular for many appliances, especially in the late 1990s and early 2000s, as it was generally rock-solid, had very mature networking, and the legal departments at the time liked the more permissive licence.

It's getting less and less common to see it, though. Sheer market share numbers mean performance, driver support, user familiarity, and companies no longer being afraid of the GPL mean that has Linux pretty much taken over.

It makes me a bit sad, but the OS on most Juniper gear is just a control plane for ASICs nowadays and NetApp has moved on to more advanced filesystems. Finding developers to write drivers/software for Linux is probably an order of magnitude easier.