Left 4 Dead

Left 4 dead has to be one the best online multiplayer experience's I've had in years. Why? you may ask, well its pretty simple, for me personally the mix of cooperative and competitive gameplay makes the game extremely enjoyable. For example the game type "Versus" places 4 survivor players against 4 special infected players (such as Boomer, Smoker, Hunter & Tank) The game type plays exactly as the co-op campaign does with the exception that the survivors are now facing real players instead of computer controlled a.i zombies, which puts allot more emphasis on strategy and team work for both sides.
For example the infected team must cooperate with one another to maximize the ammount of damage they can incur to the survivor team, this means using each infected zombies ability effectively.
Boomers have the ability to vommit on survivors which then has the efffect of drawing a horde of zombies to whoever may have been unlucky enough to get vommited on, and secondly if the boomer is shot and explodes near any survivors they also will be covered and will attract a horde, aside from that the vommit also has an added effect of partially distorting the survivng players view and also prevents them from seeing the status of their fellow survivors, such as whether or not they have been attacked by another zombie.
The Smoker is also another formidable opponent whith the ability to ahoot out a long tounge wich grabs a survivor around the neck and drags them backward into the smoker where it can rip into its prey, smart players will figure out spots on each level where they can even pull a survivor from a high place causing them to fall off a roof and die instantly, while other areas may allow the smoker to employ tactics to slow the survivng team down giving the rest of the infected team time to attack again.
The Hunter being the fastest and most agile of the special infected has the ability to jump to high places and jump off walls, the main attack of this type of infected is to pounce down onto a survivor and then rip into them, the only downside is that the hunter can easly be pushed off its prey and killed with relative ease, that is ofcourse unless the team make use of a good strategy to use the hunter, such as getting in a nice high spot possibly above a building and wait for a smoker or boomer to gain and distract the attention of the other survivors giving the hunter an open chance to pounce down on an unsuspecting survivor.
And finally there is the Tank, by far the strongest out of all the special infected, only spawning once each level to a randomly selected infected player the tank has the ability to kill all 4 survivors in a flurry of swinging arms and ofcourse the ability to pick up rocks tress and concrete and hurl tem at its opponenets, as with the other class of special infected there is a fair ammount of strategy involved in playing as a tank.
for starters the player must be careful of fire,explosives and molotov cocktails, if the tank does get hit by a molotov then its health will begin to deplete fairly rapidly givving the player less time to dish out damage on the oposing team. From experience I've found the best way to use the tank is not to blindly rush in and try to attack the survivors (usually when that happens you will have a molotov thrown at you and will die fairly quickly) so its best to hold back for a while and see what the survivors are up to and try to formulate a plan with the rest of your team, by far the best way to incapacitate a survivor is to toss a fallen tree stump or use things such as luggage carriers or skip bins at the opponent which will instantly incapacitate the victim, once that happens you can focus your efforts on any remaining standing survivors.
What I've written here is mearly scratching the surface of the game, although I will say that Versus is probably the favourite game mode for most players at the moment, although there is ofcourse the Survival mode where you are on one confined level and you must survive a continuouse onslought of zombies, tanks and all special infected, the primary goal of this mode is to try and get a gold award for each level which requires survivng for 10 minutes or more (much strategy also must be incorperated into this mode such as find the best spot on the map to defend from and the best route in which to get supplys such as health and ammo since you run through it so quickly.
To finish up what i've said, Left 4 Dead still has me hooked even though this been over a month since I've purchased the game I keep comming back to it.
The community of online players from experience is great, theres always a server running with people playing willing to give advice and help out new players and are generaly just keen to get stuck in and have a great game with a few laughs.
Fully recommend this game to anyone who is interested or simply looking for a fun online cooperative experience.