Bach, preferably short pieces I know well; failing that, Mozart, likewise. Either can serve as a pleasant diversion for those parts of my mind which get bored and need something to occupy them while I'm hacking, without requiring anything in particular from the parts of my mind which are focused on my code. (I'm not doing either composer any favors from an artistic perspective, I realize, but it's not as though I don't enjoy their work for its own sake as well.)