Zombie Panic! Source

Dive into the zombie apocalypse with Zombie Panic! Source: a cooperative, survival-horror first-person-shooter. One of the most popular Half-Life 2 modifications of all time.
Choose your team as the undead or a survivor. Enjoy their unique abilities, weapons, and challenges in this high-paced, action-packed thriller.The TeamsAs a survivor, unlock large arsenals of firearms, explosives and improvised melee weapons. Can you stop the waves of undead? Will you engage the horde head-on, or barricade and fortify while your friends play sniper? If all else fails, Panic! Then run as if your lives depend on it… for they surely do.

As a zombie, you’re free to hunt the living like the rabid beast you are. Feast upon their flesh and unlock your inner berserker. Heightened zombie night-vision gives you the upper hand in dark environments. Infect survivors as the carrier zombie (patient zero), to weaken their defenses, then sadistically watch them turn on each other. Use it all to your advantage to strategics, ambush, conquer and destroy your oblivious living counterparts.GamemodesZombie Panic! Source hosts a variety of game modes for excellent replayability. Explore the zombie apocalypse with:

Survival Mode: Put your ability to scavenge and survive to the test. Easily the top player choice.
Objective Mode: You’re on a mission. Whether it’s to escape or defeat the horde, you’d better use your brainz to find those keys, get the power back on, hack that computer, and get to the chopper! Otherwise… I think you know what happens.
Hardcore Mode: For experienced zombie connoisseurs, you and your teammates get to enjoy pitting yourselves against extra tough foes, and even rougher odds.
Assault Mode: Capture the Point. Points are awarded for completing side objectives, adding to the overall score of the players’ team which in the end decides the victor.

Zombie Panic! Source

Survival Mode: Put your ability to scavenge and survive to the test. Easily the top player choice.
Objective Mode: You’re on a mission. Whether it’s to escape or defeat the horde, you’d better use your brainz to find those keys, get the power back on, hack that computer, and get to the chopper! Otherwise… I think you know what happens.
Hardcore Mode: For experienced zombie connoisseurs, you and your teammates get to enjoy pitting yourselves against extra tough foes, and even rougher odds.
Assault Mode: Capture the Point. Points are awarded for completing side objectives, adding to the overall score of the players’ team which in the end decides the victor.