Before I begin, permit me to say that this is not DayZ; this is a mod based on DayZ. Yes that does mean it's a mod of a mod, but this no-nonsense post-Soviet zombie survival game is all the better for it.
DayZero is a free-to-play online multiplayer mod for ARMA 2 (requires Operation Arrowhead) which is basically a game in its own right. I've lost many an hour/day to the simplistically brilliant gameplay of this virtual human drama, and I've never regretted it. It's by far the most thrilling game I've ever played. Here's why.