Mood is everything when killing zombies and although I find the music of the Left 4 Dead series is awesome an atmospheric, but I like putting on other tunes sometimes to umph things up. My current favorite cd to put on to this is Fleetwood Mac's Rumours. Don't stop thinking about tomorrow, mates. 
 
What do you guys like playing to L4D?

Muse 
 
Lightspeed champion  
 
Florence and the machine  
 
^_^ 
 
We need to play together sometime

Yep! If you play the first one...
 
I don't like to play other tunes, as i like to breed that horror atmosphere.

 I don't like to play other tunes, as i like to breed that horror atmosphere. "

Queen's "Don't Stop Me Now". Cliched as it is it's still the best music you can have whilst blowing heads off with a shotgun. Ahh.

Hold Your Colour - Pendulum. 
 
Came on by mistake once I was playing the original L4D some months back, with a horde of zombies to handle I left the music play in the background and it actually sounded damn good. Especially with the sounds of hundreds of crying zombies playing a long. 

Gangsta rap. Try it.

During the times when I want to give myself a disadvantage by not hearing Tank and Witch cues, I'll listen to Ray Conniff's classic " We Wish You A Merry Christmas" album. I've been avoiding it this month, it being appropriate and all.