Brian at Clastic Detritus want to know what you listen to while writing (and ReBecca chimes in, too). My answer, unexciting as it may be, is "The various clicking and whirring of my computer." I occasionally listen to music while writing, but since I don't have much instrumental or classical music, my playlist is usually more distracting than anything else. If I do listen to music I usually put on my headphones to block some other noise out, as it's easier to ignore something familiar than something new and annoying.
If I recall correctly, though, even classical music can send me off-track at times. It can be difficult for me to catch all the little ideas that pop into my head as I'm typing away, so while I might work in a quiet setting it's pretty "noisy" in my head. The only exception I can think of is that I typically put on one of the three Jurassic Park films whenever I'm working on my book. I can ignore most of it, pay attention during my favorite scenes, and I suppose it helps to see reconstructed dinosaurs running around (as that's what I'm trying to do in a somewhat different medium). Speaking of my book, I've got to get back to working on it before all the things I learned over the past two weeks evaporate from my memory.
I tend to write better in a quiet environment as well. If I can't have that, then I go for classical music. Anything with words is too distracting. I have a really hard time writing while in lab because my desk is in the lab and everyone is talking and there's equipment making lots of noise. If/when I ever get around to writing my thesis, I'm going to have to find a quieter venue.
I don't understand these people who say they write in coffee shops. There's too much going on in a coffee shop for me to concentrate on what I'm writing!