Steam now offers early access to DayZ, the standalone game based on the Arma II zombie modification of the same name. Here's the warning about what this does and does not include: "DayZ Early Access is your chance to experience DayZ as it evolves throughout its development process. Be aware that our Early Access offer is a representation of our core pillars, and the framework we have created around them. It is a work in progress and therefore contains a variety of bugs. We strongly advise you not to buy and play the game at this stage unless you clearly understand what Early Access means and are interested in participating in the ongoing development cycle." Here's the overview of the project:

DayZ is a gritty, authentic, open-world survival horror hybrid-MMO game, in which players follow a single goal: to survive in the harsh post-apocalyptic landscape as long as they can. Players can experience powerful events and emotions arising from the ever-evolving emergent gameplay. There are no superficial tips, waypoints, built in tutorials or help given to players. Every decision matters, there are no save games, no extra lives, every mistake can be lethal. If you fail, you lose everything and you need to play again from the beginning with nothing but your wits, and your two hands. Fight the hostile environment, where every other player can be friend or foe and nothing can be taken for granted.