There has been a lot of buzz lately surrounding this mod for Arma II, similar to the hype surrounding Minecraft in its early stages of development. Unlike minecraft, this has been a game I've really been able to sink my teeth into, even though I hated Arma II.
Basically its a highly realistic, persistent-world, multiplayer zombie survival horror sandbox game. You and 50-100 other players are randomly spawned along the coast of the massive Arma II continent Chernabus (or something); there are no pre-made teams, you must decide if you will help/ignore/actively kill other players. You scavenge for supplies; you must periodically eat and drink, if you are bleeding you must bandage yourself, and if you loose too much blood you eventually start passing out and finally die. Blood can be restored slowly by eating food, or with blood-transfusions, although these can only be administered by other players.
If you die, you lose everything, and start with a fresh character. If you leave a server and come back, your character will be in the same place and have the same equipment. Zombies are best avoided; if you sneak around they won't notice you, they are attracted to loud noises, so if you fire one shot all nearby zombies will come running. You also need to be wary of other players who will try and kill you to take your goods.
I had the chance tonight to play this mod with a few other people. First we had to find each other, and since you don't start with any navigational equipment we had to use landmarks to find each other. Two of the other players using old characters already had maps, and eventually I found a map and we were able to meet up. Two of the other players who had characters already geared out with sniper rifles waited on a hill as I made my way through a town towards them. They watched me through their sights as I tried to sneak around the zombies. The day/night cycle in the game is real time and linked to where the server is located; I was playing in the evening, and the sun was going down as I crept through the town. Eventually a Zombie noticed me and gave chase; I broke into a sprint attracting maybe 6 more zombies. I had nothing but the rubbish pistol you start with, so I ran for my life as the two players on the hill picked off the zombies with their sniper rifles. We then covered the fourth player in our group as he made his way toward the hill through a field beside the town that was full of zombies. We could see him crawling through the grass as the zombies shuffled around him; he never was detected.
This was one of the coolest gaming experiences I've had in the last couple of years. Simply staying alive is thrilling, and encounters with zombies and other players are incredibly tense since death is so final in this game. It is currently in alpha and is very buggy as a result (the bugginess of the Arma II engine doesn't help), but it offers an experience unlike any other, in gaming or otherwise. The atmosphere and overall feel is a somewhat simila to Stalker, but with the world being much bigger and more open.
If you own Arma II: Combined Operations you can download the modhere
The games can also be bought for 30 bucks on steam, but I would wait until they go on sale during the steam summer sale; the game will be more polished by then anyways.