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.
Saving…
I made my first API at work last year (still making) and always saw myself looking for input on making a consistent way to return errors, with no useful input from the senior programmers or the API users. This is my second biggest problem, the first being variable and function names of course.
If I were to do anything related to HTTP, I now have something to look at.