Not to long ago Dead Trigger released for mobile devices, which impressed many gamers with it’s quality graphics. However despite costing a dollar the sales of the android version were less than stellar. While there are a number of possible reasons for the failure, the developer seems to believe it was piracy on the Android OS, which prompted them to release it for free.
Two weeks later it seems the iOS version is also listed for free. Based off the wording it seems this is a permeant drop, though I suggest you move fast incase they change their mind. While it might not be the best game on the app store, it is certainly worth a go for free.