(Alpha 21 release, PC version)
Ridiculously addictive thanks to the building and crafting systems, this game gives an excellent balance of danger and excitement. As the title suggests, with default settings, every 7 days a blood moon rises and horde night begins. The zombies become feral and aggressive, attacking you no matter where you are. You can batten down the hatches and try to survive, but the only real option is to build up a defensive position and kill waves of zombies for 6 in-game hours.
This ever-present doomsday clock means you inevitably have to build for these horde nights or be savagely eaten by the zombies. This is a fairly well-built survival game -- you have to find water, cook food, gather materials and build new items. The Fun Pimps have really done a good job at creating a feeling of isolation. You really do feel like you are on your own. Traders around various towns do help break things up, allowing you to purchase food, weapons, ammo, and other items to help you survive.
There is no story here, no end game. Just a continuous survivalism streak as the in-game days pass. The zombies have an interesting AI pattern that can be gamed, and at a certain point in your level progression, you become nearly impossible to kill. I have started over from scratch a few times just to re-live the beginning stages of the gameplay.
Some issues with the game do exist, and it seems fairly resource intensive for what seems to be a graphically modest production. These are not Baldur's Gate level graphics, but they are still effective. You can find yourself dead through various glitches or frame rate drops, and there's no real save point system.
Multiplayer is particularly fun when you have friends who jump into your game. One person hosts the game, acting like a server, and everyone else joins. This can be a problem when trying to get all friends together at once, but there are also a lot of servers that are just constantly running and anyone can just jump in to battle zombies.
Overall, while it's not the best game in the world, it certainly is one of the more entertaining games I've played in recent years. And with the first actual release on the horizon, it should be interesting to see what The Fun Pimps do with this now that they've officially released it.