It really depends on what I'm writing. I don't always listen to music when I'm actually writing (sometimes the TV's on in the background instead), but I do like to listen to certain things when I'm brainstorming. Different books have different soundtracks.
The series of steampunk novellas I'm writing and editing now are mostly White Stripes/Raconteurs/anything Jack White. Also, a bit of Johnny Cash.
A screenplay I just finished had a playlist consisting of Pete Yorn, Lady Antebellum, Johnny Cash, Michael Buble, Adam Lambert and Ryan Adams.
And another screenplay I finished a few weeks ago has a playlist that includes Nirvana, The Black Keys, Tracy Chapman, The Nitty Gritty Dirt Band, and David Gray.
And most of my novels have theme songs, too.
But like I said, I don't always listen to these when I'm actually writing. More often than not, I just have Pandora on with whatever I happened to be listening to the last time.