There are some softwares that you can install for gaining small amounts of passive income.
A very common software is Honeygain. This software uses your computer or phone to perform search queries. Who wants to give you money for that? One case are market analysts, who want to know how their company appears on google and other websites on different computers. Honeygain checks this information by using your computer. A lot of research can be done by with your help and you can earn money with it.
The software runs in the background - you don't have any inconvenience from it. It is possible to install Honeygain on different devices, which have different IP addresses. So, you can run Honeygain on your desktop PC and if you have an old phone that you don't need anymore you can leave it at your parents house to have twice the earnings. You can get ~$10 per month for every device.
With a referral code you get 5 $ just for signing up.
Similar softwares are PacketStream and IP Royal. They do the same as Honeygain and all three softwares can run on the same device. I am using them only on one device, so I can't say anything about how it works with several devices, yet. If you want to try them, feel free to share your experience by adding a comment.