Slate has a rundown on after-market earpieces for iPods (and, one assumes, those few other digital music players people might have). You can spend $40, or $70, or $330.
NewMexiKen was pleased on a recent trip with a pair of Bose headphones. (No, not the $300 noise cancelling set.) Having an over the ear model cut down considerably on ambient noise, and it appeared to keep me from irritating those nearby with the horrible, whistling-like sound that escapes from ear buds with the volume high. (A crime that should be a capital offense.)