L4D: Can't Touch This Car Alarm addon - Left 4 Dead

Set in the immediate aftermath of the zombie apocalypse, Left 4 Dead's survival co-op mode lets up to four players blast a path through the swarming zombie hordes and terrifying mutant monsters. Guide your survivors across the rooftops of an abandoned metropolis, through rural ghost towns and pitch-black forests in your quest to escape a devastated Ground Zero crawling with infected enemies.