I shopped around, then ended up with 4 (Actually 5) portable mini speakers for my MP3 players (ZEN, from Creative Lab). This is the most costly ($169), yet best sound volume and quality portable mini speakers. It is a bit heavy as compared with the others. It can be charged with a worldwide plug (and a mini USB port.) It has a volume control, It comes with a non-slip pad, it has an output port for a sub-woofer. An on off switch, a battery charging indicator light that changes color from red to green. It comes with a soft porch for carry.
The next sound volume and quality portable mini speakers is the discontinued Sony SRS-T57, the 4 AA battery compartment made it a bit bulky. It cost around $35 ?
The next sound volume and quality portable mini speakers is the Dino iSpeaker, with impressive spec, 50-20K Hz, 90 dB, small foot print, very cheap (Cost less than 10% of the Soundmatters), The sound volume and quality is good. It is a buy for the price.
The one I will not recommend is the one from Creative Lab, it fits only the Zen Mosaic model. It costs three times as the Dino iSpeaker, but the sound volume and quality is not good. I broke the shipping package, otherwise, I would return and ask for a refund.
The last one is a bulky and heavy, AM/FM radio, the Sangean PR-D5, It is a good FM/AM radio, but the sound from the aux input is not as loud as the radio. I will not recommend it for mp3 player.
In short, if you can afford the $169, get this Soundmatters speaker. Otherwize get the Dino iSpeaker (It may be model CS-SP25, developed by Cool Sources in Taiwan, made in China)