In what should come as a surprise to absolutely no one, The Atlantic has an article today noting that the best background noice for knowledge workers is in fact silence.
My own experience backs this up and so I mostly work in silence except when I have dull and repetitive work to do. In those cases I find Progressive Trance, Classical Symphony, or good old fashioned Hip Hop to be useful. The one thing that absolutely destroys productivity universally is an 'open office.'