I'm surprised no one's made a thread about this game yet.
Basically, it takes place in a world where in 2006 a second explosion occurred at Chernobyl, creating an area known as "The Zone",where serious mutations in plants and animals occur, with even the bare laws of physics affected sometimes. However, the amount of radiation can sometimes affect items throughout the zone with mysterious properties, turning them into artifact sought over the black market for millions. A few people known as Stalkers live in the Zone, looking to find these hugely valuable objects at their own risk.
In game terms, you play as a Stalker in the Zone, and have roughly 10 kilometers to freely explore. The world itself evolves around you, animals will hunt, Stalker clans will fight and quests are created at random. The game is pretty open, you can solve almost any mission many different ways. Pick enemies off from the rooftops, use silenced weapons to remove enemies, or just go in guns blazing. It plays like a FPS, but with many RPG elements. You have to mange your inventory, food, thirst and radiation poisoning. NPC interactions seem deep too.
I'm in love with the Collector's Edition box, too.
Link to box.