Currently playing this:
Rogue Survivor is a zombie apocalypse roguelike survival game. It's still in alpha but it's playable and the latest release is actually a lot of fun, it really has the potential to become something awesome. The roguelike format is pretty much perfect for this type of game.
The difficulty varies at first but can get really hard at points, sometimes right off the bat, due to the random starting positions and characters. In one of my games I woke up in the front room of a house with other people in it that was across the street from a police station full of officers with guns in a district with hardly any zombies and so I survived easily enough for the first day or two. In another game though, I woke up in a bedroom with two zombies in it and a baseball bat next to me and had to get through four rooms while dodging them before I could get outside and escape, only to find myself surrounded by even more of them. When you survive the zombies for long enough, other things will inevitably kill you, like shady government agent types with bigger guns than you. Or starvation. Or stronger zombies. Either way, you die. I can't wait for the next update.
You can get it from the developer's blog
here.