I haven't played Left 4 Dead in a long while because I wrote a crazy 8k word thesis on it at the start of the year and subsequently grew sick of the game. Which is unfair to Valve because they have been working hard at releasing new content for both L4D and L4D2. As well as making a nice online graphic novel for our viewing pleasure.
So I am going back to L4D and L4D2 as Valve is releasing simultaneous DLCs for both today! October 5th! Yay! Will tell you how they fare in due time. Until then, get on Steam and download it yerself or check out the trailer below for L4D's DLC: The Sacrifice in which one member of your hardened zombie surviving crew will face permanent and irreversible and surely emotionally charged death.
As for L4D2, they are churning out a port of L4D's No Mercy campaign. For what reason they are doing this, I am unsure, but I'm willing to bet there'll be new content to have fun with. So I'll go have a look-see and report back.