Amazon is the latest one to launch "digital coins". If you have a Kindle Fire registered to your Amazon account, that account has automatically received 500 coins which are the equivalent of $5. You can use these to purchase any apps available at the Amazon App Store including the many Photography Apps. One strategy is to use the $5 balance to try out five different 99c premium apps. Another strategy is to use it on a more expensive app under $5, still not paying anything out of pocket (for example the PicShop Photo Editor going for $4.99). Or you can use it get Photoshop Touch for $5 out of your own pocket.