FreeMyApps is a free (and LEGAL) service that lets you download sponsored apps for free and obtain points. At which point, you can spend these points to get rewards, such as itunes, amazon, and paypal gift cards just to name a few.
The service is available on both Android and IOS devices, and, as mentioned before, is free and completely legal.
After using the service, and others like it, for quite a few months now, and I have amassed a good amount of gift cards, many of them for amazon. But also a few for the google play store, and itunes.
I would have to place this app in the entertainment category. If only because the apps it provides are mostly games, some very entertaining at that. Two similar apps would be, Free slots and Free points (both only available for ios devices currently).
I have also included to the links to all 3 services on this. Just go to them on your mobile web browser for ios devices. For android, FreeMyApps is free in the google play store a standalone app.