Freespace 2 comes to mind. The whole game engine was open source not just the game itself. Looks like there is still a community building on it too. https://scp.indiegames.us/

The private space sounds interesting. I wonder how it works in practice. Can any app be stored there?

exactly, i use https://iotawatt.com/ to monitor at the breaker box. Integration works well with HA

I have the circuits for all my “dumb” appliances monitored by my homeassistant. Then HA sends notifications to my phone. And it doesn’t need to send 3.6GB to the internet to do it.