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.
S-expressions are basically directly writing the AST a compiler would normally generate. They can be extremely flexible. M-expressions were supposed to be programming part of Lisp, and S-expressions the data part. Lisp programmers noticed that code is just another kind of data to be manipulated and then only used S-expressions.
Logo is arguably a Lisp with M-expressions. But whatever niche Logo had is taken by Python now.