1. DNS resolver, like pi-hole, unbound with adguard, diversion, etc.
2. RMS server: a lot of Remote Desktop software has the option to install a listener on a low power device elsewhere on the network that can use wake-on-lan to access computers within the network without keeping everything on 24-7.
3. Log aggregator: would be useful for anyone who troubleshoots stuff regularly, but historical info of any kind can come in handy.
   Simplest form might be a scribe server. Network gear often has an option to send logs to a particular URL, so if you added the scribe server IP/port to the field you’d have historical network logs.
4. Additional loggers: could also be run on-device, such as a wifi connectivity checker, smart home or energy monitoring state data, decibel meter with USB microphone
5. RADIUS server for managing enterprise WPA keys
6. Mobile home: due to the size and power draw, when paired with a hotspot and battery the potato could be useful as a mobile service repeater, a VPN client that deploys your home services on the go (e.g. in a vehicle, hotel room, family/friends’ houses, etc) to arbitrary client devices. If you use the same SSID/PW and encryption type, personal devices would use it automatically during travel.
7. Home theater box like kodi or jellyfin client

At the level of individual apps, the list explodes. Many progressive web apps can be hosted essentially for free on the potato, so you could shunt your always-on services to this machine to allow low power states on a beefier machine. For example:

1. Network management or security software like Fing
2. Low throughput NAS or incremental backup management server like rdiff, TimeMachine, etc
3. inventory management like partkeeper, storaji, etc
4. Smart home bridges like homeassistant or homebridge
5. Bookmark aggregator or landing page like heimdall, raindrop, pinalist, etc
6. Retro game emulators or ROM libraries like retropie
7. Photo libraries like photoprism
8. Book libraries like calibre-web

Edit: list subitem formatting messed up
Edit: add common micro services, mobile deployment
Edit: add home theater suggestion
Edit: add always-on and PWA examples