Left 4 Dead. "Braaaaaiiins!"
I have no idea if this has been previously mentioned, but Left 4 Dead is clearly the best game that hasn't been released to be coming out ever.
Created by Turtle Rock and Valve it's a horror/survival FPS where you work within a team to get from point A to point B while avoiding hordes of zombies in a cityscape environment.
"Super Zombies" with different, special powers are controlled by other players and they also work hard to get you maimed by the relentless hordes of undead.
Perhaps the best feature is the use of what seems to be called "The Director". It changes the flow of every level, so you never quite know what to expect. One time you play the game and you may walk down a passage, open a door and enter an empty room. In the next game you play on that level you may go down that passage only to find it burst open before you reach it while you shoot wildly at the sudden zombie rush.
Either way I am a total fan of zombies, teamwork, first person shooters, survival and horror stories so this game represents - to me - the absolute peak of gaming.
If only those bastards would release it...
More info @
http://en.wikipedia.org/wiki/Left_4_Dead
I haven't been so excited about a release since Diablo 2. I remember the countdown to that game... agonizing!