Left 4 Dead was Valve's big shooter title last fall. It's a cooperative 4-player shooter with a survival/horror theme. It introduced procedural monster and item spawning through an AI called the "Director", and also added true multi-threading support to the ever evolving Source engine. The gameplay is fun and intense, though at release time, there wasn't much content. I pre-ordered Left 4 Dead on Steam for $45 and burned out on it in about a week. However, Valve is now back with 2 new versus campaigns and a new game mode with 16 maps in this free downloadable content pack. The new mode pits your team against an infinite stream of monsters, with the goal of surviving for as long as possible. It's probably the most intense mode yet.