Valve have officially revealed details for Left 4 Dead 2 (L4D2), as part of the Microsoft Pre-E3 Press Conference. In case you missed it, it's the sequel to last year's best-selling and critically-acclaimed co-op multiplayer zombie-shooter - and it's already looking like it's shaping up to be pretty nifty.

While it was announced as part of an Xbox 360-centric lineup, I can confirm that the game is coming to PC also, and promises to set a new benchmark for co-op action games, as it becomes one of 2009's marquee titles. Don't believe me? Check out the first-look trailer.

Gabe Newell, president and co-founder of Valve is pleased:

"A large part of how Left 4 Dead became 2008's top-selling new IP on Xbox 360 and the PC was the custom-tailored gameplay made possible by the AI Director. With the knowledge gained from creating the original, new technology, and a passionate team, L4D2 will set a new benchmark for cooperative action games."

The new game introduces the AI Director 2.0, and takes gameplay to the next level, giving the Director the ability to change world objects, weather effects and pathways, as well as tailoring sounds, effects and even the enemy population in order to match individual player performance - meaning that every unique game session can be fitted to suit, providing a "satisfying and uniquely challenging experience". There's new Survivors, new boss zombies, new weapons, new items - a much bigger game than the original, with more campaigns (both co-op and Versus), and new maps available at launch.
Download the Left 4 Dead 2 E3 2009 Teaser Trailer

Left 4 Dead 2 is set at roughly the same time as the original – just after the outbreak that turned most of the population into various zombie mutants. Players assume the role of 4 new “Survivors,” each with their own personality and new dialog. The game leads these “Survivors” through the southeastern region of the US - from Savannah, Georgia thru the bayou country, and climaxing in New Orleans’ French Quarter.

...and the best thing about Left 4 Dead 2? It's due for release on November 17th of this year.

Of all the announcements we weren't expecting, Left 4 Dead 2 is one of them. (Well, we anticipated it but not as part of the Microsoft press conference!)