Pandemic is a boardgame from Z-Man Games (Grave Robbers From Outer Space) in which 2 to 4 players co-operate to fight a global disease apocalypse.
Despite some B-grade box art this is actually an excellent game. It's hard, which is what you want from a co-op boardgame - you'll feel lucky to win one in three games. And it's fast - setup is about three minutes and games finish in around half an hour, so the best response to one of your inevitable defeats is simply to play again.
The aim is to find cures to four diseases ravaging the Earth before they wipe out the human race. Each player takes one of five roles. The medic can treat infections more effectively, while the scientist finds it easier to develop cures. Other roles include a researcher, a dispatcher and an operations manager.
Infections are depicted on the board using coloured cubes. At the end of each player's turn, more cubes are added to the board, and if more than three cubes build up in a single city an outbreak occurs, causing infection to spill over into neighbouring cities. A total of eight outbreaks over the course of the game results in defeat, and as outbreaks can cascade it's possible to lose the game in a single turn.
The game's roughly the same difficulty and length with any number of players, which is great, and while a lot of the setup is random you never really feel like you've experienced an unusually hard or easy configuration.
There's really no other games out there quite like Pandemic, and the combination of uniqueness and quality makes it a worthy addition to anyone's collection.