A friend of mine recently turned me onto a free online FPS called "Combat Arms";
It currently has three 'zombie modes' that other players have described as reminiscent of other games like Left4Dead (I'm taking their word for it since I never played L4d);
The difference between this game and others is that it is 100% free*.
There is no initial purchase or monthly fee to play;*
* = It's a micropayment model of online game (similar to games like maplestory and other nexon games); which means there is no initial or monthy fee's associated with it, but it also tends to mean that the best equipment can be obtained through real world money used to purchase ingame gear and other items.
I haven't spent a dime on this game so far, and I exclusively use the 'gear point' (GP) system to purchase gear to outfit myself (GP is earned through gameplay when the match ends). It's free, and it's not terrible.
One zombie mode is player vs player; (survive or eliminate zombie or infect and zombiefy the remaining survivors).
Two other zombie modes are player vs. AI; (survive oncoming waves of zombies. Some zombies have different traits, are faster or slower, etc).
Quarantine Mode (Player vs Player = 'Humans' vs 'Infected'):
The match begins with player spawn, immediately followed by 15-30 seconds of opportunity to either group together or separate, to hide or to find a defensible position; after which time (20-30 seconds after the match has begun) the 'host infection is selected' (ie; those players who will be the initial randomly selected infected, who will immediately infect any remaining human players with their touch).
The initial host infection begins with 500 health (as opposed to human 100 health), something like x5 sprint stamina, and have what is effectively a 'one hit kill' with a melee attack; which will infect any remaining human players and cause them to join the zombie army and infect yet others.
Human players who are infected after the initial infection begin the round as human players wielding the basic assortment of equipment, but after being infected they are cabable of only clawing melee attack; If you are a 'host zombie' you have 500 hp (as mentioned above), if you were infected by a host zombie you have 300 health and x3 sprint stamina.
Any non-infected humans will have to either group together after the initial outbreak (random selection at the beginning of each match), find a good place to hide independently from any group (because the best places to defend happen can quickly become death traps if the infected manage to get a foothold and start infecting more and more defenders), or be clever enough to lure the infected into traps, deployed explosive, and massed machinegun fire.
In the overdose map at least, there are 'secure areas' that the humans can retreat to, but they do not become unsealed until a few minutes before the game will run out of time. Even these barricades can be overcome, but it will take time and force of numbers to bring them down before the round ends.
Short Review of play:
The 'Infected/Zombies' in this mode of play are recognizably something more akin to 28 days/weeks later series of movies and 'infected human' genre, and less of the completely zombified 'dawn of the dead' or 'the walking dead' type of zombie.
They are fast, intelligent (they are being played by other devious human players after all), and once a human is infected he immediately begins craving flesh.
'Infected' players are difficult to kill, and It's been truly amusing playing quarantine. Whether watching groups of human players scatter in all directions like bowling pins from a single infected rushing them while they're caught in the open, or seeing everyone trying to flee their barricade like rats from a sinking ship after their buddies have become infected is always entertaining; no matter whose side you happen to be playing on.
Quarantine Mode played on 'Overdose' map Gameplay vid:
Cabin Fever and Blacklung (fireteam/coop = human vs. computer):
The match begins with player spawn and less than a minute to take defensive positions to defend a static location against multiple incoming computer controlled enemies.
After the round has begun, zombies will spawn at the edge of the map and slowly shuffle towards the players, and attempt to overwhelm them. In the course of a game it is not uncommon for experienced players to kill hundreds of zombies before a match has ended.
There are a variety of different zombies in this mode of play, from the basic shuffler, to a female zombie who moves faster and claws harder, to a suicide zombie with a bomb strapped to it's chest, to a blue version of the incredible hulk zombie.
The goal is to survive all rounds and escape before the time limit expires or everyone is dead.
Note - Human players do not rise and crave the brains of their fellow players in this mode. All the zombies do to you in this mode is kill you.
While less interesting than quarantine, working with a group to survive endless waves of slow moving zombies can be fun too. The big change is the lack of any dynamic strategies, since the computer controlled enemies are very predicable (unlike human players) and fall like flies compared to human infected.
It's also a useful mode to gain experience and gear points (GP) in;
http://www.youtube.com/watch?v=0IR6JPh0 ... re=related
Cabin Fever(CF) (skip to 1:00 in to bypass intro):