Varies, but it's not much different that what I listen to when I'm not programming. My tastes bounce around from day to day, but I tend to classical (mostly chamber music), old metal (original Sabbath era), "classic" jazz (coltrane era), celtic, "old time" banjo, and recently dubstep (Skrillex).

Mostly though... nothing. When I listen to music, I really listen to it, so when I'm concentrating on coding, I usually have it as silent as I can make it.