Ear monitors +1
Best bit of bike kit I've bought in years. Music is crystal clear and can be heard on low volume without having to compete with wind noise. It sounds like the music originates inside your head.
They are expensive, can be prone to wires breaking and can sometimes be uncomfortable.
I've just bought a new set from Ultimate ears which have kevlar reinforced wires and a softer compound which fits the ear canal better, still expensive though!!