UberGeek-"Anything that doesn't have noise reduction. You need to be able to hear that noise. I just use a cheap $5 pair of earbuds, in my right ear (Keep the road-side ear clear)."
Exactly what I was going to write, very, very good advice. One of my personal bugbears is seeing riders on the road with headphones in both ears, it's not big, it's not clever and in many places it's quite rightly illegal.