For me, the music/writing interface can be broken down into two groups: what I listen to while being inspired, and what I listen to while writing.
While being inspired, I'm usually driving, and I'm usually alone. Something about the combo of music, driving and solitude (enhanced by puffing on a cigarette) gives me good ideas. At that time, the music is something that relates to what I'm dreaming up. I can name Queensryche, Led Zeppelin, Loreena Mc Kennit
and Abney Park as common bands, but it's usually all over the map. I find lyrics more inspiring than the music, though there are exceptions.
While I'm actually writing its a different story. I want something to fill the silence, something pleasant to my ear but not too catchy to distract me. For this, Bad Company fills the bill. I don't know why, but something about it just works as writing background music. For my more depressing pieces, I use Stabbing Westward for the same reason.
I can't, however, imagine writing without music.