I just read about this in a PCG magazine that was randomly bought for me. It caught my eye with "Exclusive #1! VALVE'S SECRET ZOMBIE SHOOTER".
Of course VALVe was going to mean HL engine so I eagerly read on.
'Left 4 Dead' looks absolutly amazing, thats all that can be said. Its not some mindless "Headcrab bash". Its not even set in the HL universe. You play as one of four survivors in a city flooded with infected zombies. This is a Multiplayer Co-op game, and whats better, you can play as either one of the four survivors or as one of four zombies. There are no respawns, and each 'level' it is said can last from 40 mins to an hour.
Play with friends, or online, or on your own (with high level AI taking control of the unused players). The AI is also somthing to take note of, the monsters look and sound (from what I have read) terrifying.
You have your normal all out zombies, fast zombies (think 28 days later, not HL2), fat zombies that can vomit all over you and explode in a wash of vomit when they die, zombies with snake like tounges that slither around and get you like a barnicle. 'Hunter's which are much like the lickers from resident evil except they have no tounge and they are more agile but weaker.
This sounds fantastic, and what is more there are no scripted sequences, every time you play a level it will be different. Enemies will be in different places and if you play as a zombie you can 'herd' the masses of AI zombies to the players. From what I have read and seen from the screen shots this looks totally amazing. DEFFINATLY one to watch. I eagerly await its arrival. Watch this space.
http://turtlerockstudios.com/l4d/