Home Assistant is a Python-based hub that can run on a Raspberry Pi, a small x86 box or a virtual machine. Its integrations talk to devices over Zigbee, Z-Wave, Matter, Wi-Fi, Bluetooth and MQTT, as well as to cloud services, and present them in a single dashboard with automations, scenes and voice control. Because state and logic live on the local machine, automations keep working when the internet is down and no usage data leaves the house by default.
The trade-off is maintenance. Users take on updates, backups and the occasional breaking change in an integration, which commercial ecosystems hide. For many it is the reference example of open-source IoT; the wider context is in The smart home, explained.
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