Walking Dead: The Game, Episode 1 (free) is currently free in the App Store for a limited time. Nothing says Merry Christmas like a good zombie horror game. The game is based on the original award-winning Robert Kirkman comic books, the basis for the current TV show. This is the first of 5 episodes (the other 4 are available as in-app purchases). MetaCritic gives this game a 86/100 rating.
Sample of gameplay: