I still get royalties from programming books I wrote about ten years ago - the content has been updated by other authors in the mean time, and the corresponding

I still get royalties from programming books I wrote about ten years ago - the content has been updated by other authors in the mean time, and the corresponding royalty split has declined, but I don't do anything active either. On the not-quite-as-passive side, I have an iPad game that I had basically left out in the Apple App Store for several years, where it sold with no involvement or investment on my end. Only recently, Apple required all app developers to resubmit their older apps or face delisting - in my case, it involved updating cocos-2d to support the latest ARM architectures, and making a whole lot of changes for everything that had happened in iOS in the past couple of years. I also run a small API-as-a-service, which runs on reliable infrastructure, and doesn't take much real work to monitor and/or run.