AYL (TM) Portable Mini Speaker System for PC / Phone / Tablet / Apple iPod Touch / iPhone 4 / iPad / MP3 Player (Black)
This unit plays surprisingly loud for an unpowereed speaker.  The highs are relatively clear and discernable.  The lows....well you need power to reproduce low frequencies.  So if you want ambient music in a quiet place, this is a nice solution.  If you want bass and higher fidelity, spend a good deal more money and expect to use more space in your backpack.