RPi 4:

  - VPN server (Wireguard)
  - Network ad blocking (Pihole)
  - DNS resolver (unbound)
  - DynamicDNS client
  All running in Docker containers.

RPi 3:

  - Home Assistant for home automation.

I also have a Pi Zero that I intend to use to drive an e-ink screen for a wall-mounted display, but I haven't set that up yet.

Previously I've also used the RPi 3 running AirPlay and hooked up to external speakers as a poor man's Sonos.