I have been using Pandora Radio at work for over a year now. I have "trained" one station such that I skip tracks only about once or twice a day.
It helps me focus. Perhaps it blocks out other distractions, I really don't know. All I know is that I have much better focus when the music is playing.
As for what music: it's quite a wide range. New Age/Celtic/Progressive Rock/Folk/Folk Rock, etc..