Some services I have liked in the past are:
- Syncthing if you have some storage space
- A backup server
- Websites and Demos
- i2pd and your own i2p website/blog
- Monero mainnet and Haveno
- Crypto Mining to support the network not to make money (Limited to a small portion of your resources)
- Game Servers
- VNC on some virtual machines for a networked device
- A remote containerized development environment
But there are all kinds of things you can do with some continuous computation and networking!