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 use xmpp. It happens to be a great fit for a private family messaging service. Good interoperability between modern clients. I get that “nobody uses it” is hyperbole, but the internet is a big place and there is room for services without mass market appeal to thrive.