A stupid app, called That's Not Funny, that I wrote in 2008 or so to teach myself Android programming (when v1 came out) continues to make around $40 a month from ads.

https://play.google.com/store/apps/details?id=com.vbrad.andr...

I wrote it, released it, then to my surprise, it got a pretty massive amount of downloads. Over the years, I've updated it to new versions of the OS, but very minimal work.

Not a lot of money, but it wasn't a whole lot of effort either. It covers the internet bill.