For 10 points, name this Valve series in which four survivors battle zombies.
One game in this series released a downloadable level called "Crash Course" in which the player is stranded by a helicopter crash. This series replaced fixed spawn points with an artificial intelligence system named "The Director." The newest game in this series introduces opponents in hazmat suits and (*) riot gear. Opponents common to both games in this series include the sound sensitive witches, the long tongued smokers, and the explosive boomer. For 10 points, name this Valve series in which four survivors battle zombies.
One game in this series released a downloadable level called "Crash Course" in which the player is stranded by a helicopter crash.
{Left 4 Dead}