I just picked a pair of these LG Portable Speakers up on Ebay for $5.99. They aren't much bigger than a cell phone and plug into any audio device or cell phone that had a headphone jack. The speakers have a built in chargeable battery that takes a couple of hours to charage and the volume is controlled by your device.

I was totally suprised by the sound. The speaker may be too close together to enjoy real stereo sound but if you are in a pinch and need some loud music this does the trick. I hooked them on my golf car with my Sirius Stiletto and was rebuilding a fence on the weekend and took them out there. Saves lugging around a portable boombox and means you don't have to use an fm transmiter with your Sirius. Perfect.