Basically, it's the second half of ActivityPub that's for mapping an instance to clients. Most platforms on Fedi use bespoke APIs or copy Mastodon, but C2S is kind of more fluid and lets you build custom experiences and logic that hooks into it?

What's cool about Vocata is that you can kind of just make up the vocab and activity you send out the Outbox. Vocata just shrugs and says "whatever, that's valid."

It could be brilliant for prototyping.

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.