Nostr is more of a protocol / network in and of itself, as is Scuttlebutt. Both legitimate efforts in their own right, but not quite Fediverse in the traditional sense. Though, with bridging, it's getting harder to tell where one starts, and another stops.

Regardless, I would love to start a list of really esoteric Fedi software like what I have above. I know there's more, it's just harder to dig up.