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.
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.