Zombie Gunship
Fun little free game for the Android and iOS. Basically it takes that "Death From Above" mission from
Call of Duty 4: Modern Warfare (2007) and lets you shoot zombies instead of Russians. You fight off wave of the undead while trying to protect civilians for long enough to earn you more money to spend on upgrading your weapons and defenses and then you do it again. It's stupidly addicting.