Valve is a video game developer based out of some place you're never heard of in Washington. They show a passionate love of violence, zombies and physicists.
Valve kicked off with a little game called Half Life and things sky rocketed from there, the game was an instant success and continues to sell to this day. In the game you play as Gordon Freeman, a mute scientist who starts off a shitstorm when he puts some kind of alien sample in what appears to be three magical barrels that shoot lasers.
Aliens then start to appear out of nowhere and kill all of the scientists in the underground facility, known as Black Mesa. You, as Freeman, then are forced to fight to stop the alien scum and all seems well by the end of the game.
In Half Life 2 it is revealed that all is not well, in fact just the opposite. The aliens changed from monsters with many eyes into human shaped police called the Combine. The game follows you moving towards the rebel base so you can fight the Combine with your rebel comrades. Of course you never make it there and the journey lasts through two more games, and when you finally get there you're told that they followed you.
Everyone seems very surprised at this even though you're left a trail of dead Combine for a couple hundred miles. The resolution to the series has yet to unfold, Valve is too preoccupied with zombies.
Which of course brings us to zombies!!! Valve released Left 4 Dead and it's sequel in close proximity with each other. In both games four survivors fight off endless hordes of zombies so that they can be rescued by extremely incompetent military personel, slow pilots, etc.