Alot online stores offer Gift Cards. Probably you have seen or own gift cards for Google Play or Apple Gift Card. Mobi.to for Android (and iOS) found a great way to pay you for installing apps that you realy need.
The app is basicly a list of apps and for each app you get coins. All coins go to your personal wallet. When you have enough coins you can exchange them for Google Play, Apple Gift cards, Amazon or Facebook cards and even cash on Paypal.
Well this is a quick way to get some cash and find free apps.