None. Music by it's very nature is designed to draw you into it's world. If you want background nose, use background noise.
Absolutely second that. I never understand how people can concentrate with their headphones listening to music. For me it is only helpful for recreation.
I'm listening mostly to ambient and instrumental music. MusicForPrograming has a great selection of them: http ://musicforprogramming.net
Electro-house or hard trance. Something with a solid repetitive beat and minimal to no lyrics (occasionally those lyrics may be in a foreign language).
Nick Cave's just released "Ghosteen" is perfect. Quiet, beautiful soundscapes.
Nightcore white noise wardrums whatever is playing as long as it's not my native language