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!