Depends on who I'm writing as.

I don't really write by myself. I roleplay, but I definitely classify it as writing just the same. So, it depends on the character I'm writing for. Elena Incatasi, which is my bitch vampire queen who doesn't take no for an answer, gets Avenged Sevenfold as the creation for my muse. If it's a quieter, softer character, I'll listen to something classical. Each character has it's own "playlist" that helps me get settled into the mood of the character.