Age of Zombies, a mobile port from the PSP, is free today on the Amazon Apps Store for all Android users and is a nice counterpart to the free iOS app that was also announced for today. Keep in mind a wi-fi connection is required to download the game, so even if you snag it using your carrier's data package, you may have to wait to actually get it onto your phone.
The mobile version of Age of Zombies doesn't have the tightest control but it is free, and still fun. You play as a man named Barry Steakfries, in a game that does not take itself seriously at all, killing cavemen, dinosaurs, mummies and ninjas that have all been turned into zombies. I'm not sure I would have paid for this, but it is certainly enjoyable for the zero dollars I spent.