My side project is: "How to Make an RPG" (http://howtomakeanrpg.com/) which I released in June.
It's a collection of code samples, art and digital book that shows the reader how to make an old-school, Japanese-style RPG. So, it's super niche! I wrote a little about my process here:
https://medium.com/@DanSchuller/my-first-side-project-part-1...
There was supposed to be a second part to this article but I haven't written it yet.
It's been over $1000/month very comfortably so far but it is trending down. This isn't uncommon for this type of project - there's often a spike followed by a slow decline.
Still, for the last three months I haven't actively worked on it and it's still sold well. I've moved country and been finding a job (all sorted now), so I haven't had much free time.
I'm not really sure where is good to go after this project. For now I'm building on the base the book introduces, just for fun.