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.
Installed RabbitMQ for use in Python Celery (for task queue and crontab). Was pleasantly surprised it also offered MQTT support.
Was originally planning on using a third-party, commercial combo websocket/push notification service. But between RabbitMQ/MQTT with websockets and Firebase Cloud Messaging, I’m getting all of it: queuing, MQTT pubsub, and cross-platform push, all for free. 🎉
It all runs nicely in Docker and when time to deploy and scale, trust RabbitMQ more since it has solid cluster support.