* UrBackup for all of the host systems in the home to backup to
* Plex for home movies
* HomeAssistant
* OpenThread Border Router
* Zigbee2MQTT
* Matter Server
* PostgreSQL (for HA)
* InfluxDB (for long term statistics from HA)
* EMHASS (linear optimizer to maximise profit on my Solar PV + Battery system)
* Minecraft server for the kids
* Mosquitto MQTT broker
* TeslaMate for car data
In my case, the heaviest use is HomeAssistant - every light in the house has smarts, either directly, or through smart relays. Telling my energy provider how much charge to add to the EV's so they schedule it and I only pay 7p per kWh during the dispatching windows. Managing energy flow in the house (charge battery? discharge battery? only charge battery from solar that would otherwise be curtailed due to a 5kW export limit) etc. etc.
It's running in a short depth 2U rackmount chassis inside my network rack, with 6 4TB drives running in RaidZ2 and offsite backups which are aided by a 1Gbps symmetric FTTP connection.