A free mod, if you have a Source Engine running game, like Hl2, CSS etc.
Thanks to Half-life 1, Zombie Panic was spawned and thus after Hl2, a source engine running ZP was made as you see here.
Mainly really only a multiplayer game, players start out having to survive or escape the map, following objectives.
Soon after round start a random player will be picked to become a "special" zombie, one that has a larger chance of infecting players. When a player dies, they become a zombie themselves and must help kill out the rest of the surviving players.
Around the map you can find different sorts of weapons, ranging from melee to long/close range based weapons. The game also uses a ammo system, which you can decide what kind of ammo type and how much you want to drop. You see, the more you carry, the more burden you get, and become slower. You become a easy target for a swarm of zombies.
Items also include health packs, pills, body armor and wooden planks you can use to board against windows or doors to keep out zombies for a small amount of time.
The mod has a thriving community with a decent amount of servers, and there's loads of custom maps incase you become bored of the usual classic maps.
Overall I would highly recommend this mod if you enjoy zombie games.