Zombie Panic! Source

Dive into the zombie apocalypse with Zombie Panic! Source (ZPS): a cooperative, survival-horror first-person-shooter. A fast-paced standalone modification. Choose your team as the Undead or a Survivor. Enjoy their unique abilities, weapons, and challenges in this action-packed mod!
The Teams
As 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.
The Gamemodes
Zombie 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 under a timer.

Custom Made: Enjoy all kinds of unofficial gamemodes by community modders that push the game to its limits using AngelScript or SourceMod plugins.

Community Servers and Workshop
Zombie Panic! Source wouldn't be half the game it is without nearly two decades of community content and support. Over the years this has included countless custom maps, Workshop add-ons, and community servers running all sorts of modified gameplay that might fit your style! Official, custom-made, or both - pick your favorite!