There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
Would you be interested in a similar protocol that supports more things (and is IMO easier to set up)?
I’m not actively looking but please do share references! Other people may read this and they may want to know too. Perhaps I’ll jump back in the rabbit hole at some point too 😁