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.
org-mode is awesome for many reasons, but the similarities/overlap with markdown are an incidental benefit. I wouldn’t learn org-mode for that reason, however there are many other good ones that make it worthwhile. I’ve been using it for years for my own project management, tasks tracking, notes and many other things - it’s one of those rare tools that can do many things incredibly well.