If it helps, here’s a (partial) list of ActivityPub software - I’m not sure why it doesn’t include things like Friendica or Owncast: github.com/BasixKOR/awesome-activitypub

But regarding your question, the first example that comes to mind is PeerTube. Not only does it look to me like it was designed from the start with federation in mind (I don’t know this for a fact though), but it also seems pretty innovative with its use of peer-to-peer video streaming. This 2 minute animated video does a good job of explaining what it does: framatube.org/…/217eefeb-883d-45be-b7fc-a788ad850…

Owncast seems somewhat similar.

It seems that most Fediverse/ActivityPub software is a “twist” on something that existed previously, but there is still a lot of innovation going on, instead of pure copies of existing centralized platforms.