Check out this Spring USB Speaker which is compatible with any Media Playing device. They are pretty small but powerful, they give an output of 2Wx2. It comes with an 3.5mm headphone jack and also a USB connector so it will work with even your laptop.
There is an integrated li-ion rechargeable battery that gives 4 hours of life from one single charge from a USB port. The Spring Speaker can be yours for only £14.99 over at Thumbs Up UK.