I will use my first one to power a hard drive that I leave with family or friends. They'll have a small box (mind the inclusion of an external hard drive, still a relatively small box) with my off site back up, and I could host the same for them.
This is after a failed plan to use it for Android TV (my girlfriend made the mistake of picking a WebOS TV and I made the mistake of thinking it wouldn't be so bad). The old one was just a little too slow and only did full HD (honestly, it's fine, but if you have a 4k TV it feels kinda silly). Now that the pi4 has a bunch more power and can do 4K at 60Hz, there is another chance for this!
I'm also toying with the idea of using it for sensors. Battery powered air quality sensor to see along my walking route (there is a narrow, busy car passage that I'm curious about), or maybe measuring things like electricity or heater usage in realtime. Having a graph showing you when it gets used a lot might help identify some easy wins, since walking to the basement to check the meter is a little cumbersome. But those are just ideas.
Regarding your mutual backup idea, I love this and I've wondered why this hasn't taken off as a Thing in response to how many of us are untrusting and DIYers in this era of cheap storage and bandwidth.
Indeed! I'm not sure why I don't already have three hard drives from friends, it's 10x cheaper than S3 or Backblaze B2 or any such thing. I've had this idea for years, but seeing that someone else built exactly this[1] while also having time for it soon gave the idea another push.
This is after a failed plan to use it for Android TV (my girlfriend made the mistake of picking a WebOS TV and I made the mistake of thinking it wouldn't be so bad). The old one was just a little too slow and only did full HD (honestly, it's fine, but if you have a 4k TV it feels kinda silly). Now that the pi4 has a bunch more power and can do 4K at 60Hz, there is another chance for this!
I'm also toying with the idea of using it for sensors. Battery powered air quality sensor to see along my walking route (there is a narrow, busy car passage that I'm curious about), or maybe measuring things like electricity or heater usage in realtime. Having a graph showing you when it gets used a lot might help identify some easy wins, since walking to the basement to check the meter is a little cumbersome. But those are just ideas.