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.
Since nobody’s brought it up: MQTT.
It got pigeonholed into IoT world, but it’s a pretty decent event pubsub system. It has lots lf security/encryption options, plus a websocket layer, so you can use it anywhere from devices, to mobile, to web.
As of late last year, RabbitMQ started suporting it as a supported server add-on, so it’s easy to use it to create scalable, event-based systems, including for multiuser games.