Some ideas of what you could do with that old PC
Without preaching the Linux vibe
I suspect many of us have a spare computer laying around. I have a few, it's one of the joys of being a programmer that we occasionally acquire very useful things.
I’m mostly focussing in this article on projects for tower-type PCs, notebooks tend to have less longevity. One of the best ways to develop as a programmer is to make things. Sometimes the hardest part of a new project is coming up with the idea in the first place.
Home intranet page
IIS is easy to install on Windows PCs, apache servers on Macs, and Linux boxes. Having a home intranet site on the box can do wonders for home communication. without the worry of exposing your site to the outside world. You can add a wide variety of projects to your home site.
House Blog — A kind of home newsletter. A wall to put up the pictures of the latest craft project. When granny is coming to town. Or this week's chore rota. There are plenty of things you might blog about if you knew the word wasn’t going to the world.
Shopping List — a page for you to get a list from everyone what they want in next week's shopping, then generate a shopping list. Bonus points if you can hook it up to your supermarket's API to do the shopping for you.
Chores — Reinvent Trello, or not. A page to organize the week's chores, perhaps have a signup sheet for pocket money attached.
With many developers dabbling in financial APIs, a home server can carry out worker tasks that automate things.
Home stock tracker. Use free APIs and web scraping to analyze stocks of interest, and store information in a db, for feeding into your day-trading strategy.
Set up an Arduino with a raspberry pi to record weather or fish-tank state data. Use the home server as an API server to automate the collection of data-logging data. Cool to do with older kids who are getting into automated science experiments.
Home Web Server
If you're wanting to get a better idea of how the internet works, or simply want to prototype your new idea — connect your web server to the outside world. There are some risks in doing this, and you should read up on application security before you try. The golden rule is never to put anything on an open server that you wouldn’t want to be attacked. But hey this is a spare computer that you’re not using for anything else. Learn about DNS, and publish a site. Even if it's only a sanitized version of your CV. The other bonus is you get to control all your trackers and cookies, and can’t get put in Facebook jail for what's on your own computer.