Cash em all app: This is an app which advertises mobile games and pays you money for playing. The nice thing about it is, that you don't need to play the games. Just install the games and start them on your phone - then put you phone aside and let it earn money for you. You get paid with coins, which you can cash out via Paypal. Cashing out is very fast - usually I have the money on the next day!
I used to keep the games running while I worked or watched TV (without playing the games off course). This way I got 40 € as an additional source of income within two months. Sadly, the earning rate decreased after the first month, which is why I'm not using it anymore. However, the earning rate is increasing again, so maybe I can harvest a bit more with it in the future.
If you use my link, I get some of your earnings and you get a part of mine (hopefully, I will use the app again in the future).
EDIT: The app changed to an offerwall model in May 2021. So instead of just downloading a game and keeping it open, we now have to perform specific tasks, like play the game until we reach a certain level.