Zombie Panic: Source is the sequel to the Half-Life mod Zombie Panic. Zombie Panic: Source has much more enhanced graphics, a larger variety of weapons, and has much more team-oriented game play than its predecessor. The addition of a new zombie, the Carrier, created a more balanced game. The mod can be downloaded and played for free on Steam as long as the Steam user has Source SDK, and Source SDK 2007 installed, both of which are free as well.