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.
I wish there was a good open standard for task management or todo list.
I know there’s todo.txt, but it lacks features like dependent tasks, and overall the plain text format limits features and implementations.