What sets one online music store apart from another? All offer per-song downloads for around a buck, albums for around ten. The least imaginative among them simply cut a few pennies off the per-song price tag, and call that differentiation. Others are innovating themselves into the game by developing community features and — most importantly — new ways for users to discover music they’d otherwise never hear. To learn more, we dropped ‘em each a line and asked them about their strengths, their philosophies, and where they see the music market headed.