Cooperative games

In Cooperative games the players unite to defeat the game, and stand to win or lose together. Some games introduce a 'traitor' element, where one unknown player is secretly trying to defeat the rest. Here are some great co-op games;

Arkham Horror.
The players take on the roles of investigators trying to defeat the diabolical servants of 8 Ancient Ones, based on the Call Cthulhu mythos.

Pandemic
Four diseases have broken out in the world and it is up to a team of specialists in various fields to find cures for these diseases before mankind is wiped out.
Players must work together, playing to their characters' strengths and planning their strategy of eradication before the diseases overwhelm the world with ever-increasing outbreaks.

Shadows over Camelot
Shadows over Camelo is a unique collaborative game featuring a malevolent twist! As the incarnation of the Knights of the Round Table, you join forces against the game itself in an attempt to protect Camelot.Your victory hinges on the successful completion of legendary Quests, such as the search for Excalibur, the Holy Grail, or Lancelot's Armor; the tournament against the Black Knight; and numerous wars against the Saxons and Picts.
But beware... all is not as it seems among these noble Knights. One of your number might yet turn out to be a traitor-in-waiting, biding his time while sowing havoc and destruction from the Shadows!

Battlestar Galactica
Battlestar Galactica: The Board Game is an exciting game of mistrust, intrigue, and the struggle for survival which places each player in the role of one of ten of their favorite characters from the show. Each playable character has their own abilities and weaknesses, and all must work together in order for humanity to have any hope of survival. However, one or more players in every game is secretly a Cylon, and wants the humans to perish.