I've learned a lot about MQTT while setting up Home Assistant, and Mosquitto is a pretty important part of my Home Assistant installation. I use it to control all of my Zigbee devices via "zigbee2mqtt" (buttons, switch modules, etc.), and also a smart kettle running Tasmota [1]. I also added a little patch to Tasmota [2] so that my kettle responds with `418 I'm a teapot` [3]
[1] https://tasmota.github.io/docs
[2] https://github.com/ndbroadbent/Tasmota/commit/bbcf57faffbf5b...
[3] https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/418