Studies have shown that classical music tends to improve concentration, and rock music tends to disrupt it. Something to do with how the format of the music builds logic connections in the brain. Given, the latter is probably better at waking you up (unless you listen to Sousa), but I don't really see how it could improve your coding.
Rap is about as useful as shooting yourself in the head.
Comment on Re: Music Soothes the Savage. . . code?
The occasional disruption is a good thing for some people. If you're prone to getting into intellectual ruts, and need to shake up your complacency to get a fresh look at code and get interested in it again, then disrupting influences like rock might be exactly what you need.
For my own part, I listen to a wide range of things (probably two songs in a thousand being "rap" of any description, and certainly not any gangsta rap or hip hop). I set XMMS to random and let it play. There's a great deal of rhythmic, complex, energetic stuff to keep my focus at its peak, with occasional jarring changes in musical style to help keep the code from turning into a homogenized blur.
I find, though, that Wagner is truly awful for coding, as is Tom Waits. The former just makes me want to watch Looney Toons, and the latter makes me wonder at my own sanity for having such odd sounds on my playlist.