When I ride, I usually listen to my iPod. Here in FL it's legal to have speakers mounted in your helmet, it's not legal to use ear buds. (the speakers need to be 'mounted' in the helmet, not your ears.
I've seen lots of folks say it's bad to listen to music, but I find once I put a helmet on I can't hear much of anything else anyway, and I don't play the music at full volume, so I'm still aware of my surroundings.
As for what I listen to, it's a very mixed collection, but mostly classic rock, some Motown, and others.