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.
That sounds interesting, would most likely not be very popular with lots of people and a pain in the butt to implement but interesting.
There’s a cool video from In a Nutshell about it some years ago.