Sony's spunky little SRS-T57 travel speakers are among the most compact and affordable models we've seen. With its stereo speakers folded in, the unit measures a scant 2 by 3.5 by 4 inches. To play music, you swing open the speakers to give some miniaturized semblance of stereo sound. The T57 weighs 12 ounces when loaded with four AA batteries, but you can also use the optional AC adapter (about $15). Since the speakers don't have any sort of volume control, you'll have to change the level on your player or laptop. The T57 retails for $50, but you'll likely find better prices online.
The SRS-T57's sound quality wasn't great; even by portable-speaker standards, its output is awfully thin and weak. Sony claims the T57 features Mega Bass, but it sounded more like Micro Bass to us. Wilco's trippy Yankee Hotel Foxtrot CD was steely and harsh at moderate volume, but it grew more tolerable when turned down to background level. We conducted our initial listening tests with an Apple iPod. Switching over to our made the T57 sound--gasp--even worse, though it still creamed the pathetic speaker set. Even after the T57's batteries are spent, the speakers can still play passively at much lower volume, using power from the MP3 player itself.
The T57's sound is really, really small. If portability is all that matters to you, then this system might fit your needs. But if you care about sound, we'd strongly recommend spending a little more and getting either or speaker sets.