A new app called 'Flag' is exploding on Kickstarter today, and it's based on a very simple premise. The app will print and mail you museum quality, 220 gsm prints of 20 photos per month absolutely free... just as long as you allow them to place advertisements on the usually-blank back side of the photo.
No catch, when usually there tends to be one. Flag will not disappoint you, one bit. You receive 20 photos per month at no charge, and maybe the only real catch would be that they have to place an ad on the back of that photo. But anyone could live with that.