If you have an always-on internet connection and a spare PC, you've probably thought about installing a server on it at some point. It's a good way to learn about how internet things work, after all. And if you use Linux rather than Windows, you don't even need to pay for an operating system. Plus, it'll happily run on an old, slow machine. Even an old laptop.
But once you've decided you want to run a server, you then need to think about what the server is going to be used for? Do you want a web server, or an FTP-based file store? Or your own social network? Or your own dropbox clone? Or a complete online office suite to rival Google Docs.
To help you choose, here's a list of hundreds of products that you can run on a server. What's more, they're all free of charge and open source. You could even run more than one of them, if you want. The list covers everything from file stores, web servers, groupware, video streaming, and dozens more categories besides.
Remember that running servers is not for the faint-hearted, so you'll need some technical knowledge to get these up and running. And you'll probably also need to change some of the firewall settings in your router too, if you want them to be accessible from outside your own LAN. Make sure you understand the consequences of doing so. Or just sign up for a free trial on Microsoft Azure, set up a server on there, and you can play without having to worry about security.
You'll find the complete list at https://github.com/Kickball/awesome-selfhosted