Make your MP3s HEARDI love music, and I believe it should go everywhere I go. Now if you already have some nice speakers for the portable music player of your choice, or maybe even a regular boom box with an input on it then this post isn't for you.
Cambridge SoundWorks PlayDock MP3
This speaker unit will turn your iPod or other mp3 player into a boom box! It boasts a 15 watt powered subwoofer with 4-inch driver and approximately 10 hours of playback time. It'll even recharge "select MP3 players while AC powered". It's $199 on the Cambridge Soundworks website, which appears to be the only place it's available.
This next guy is one I can personally vouch for because I own one. It'll just about fit into a nice large cargo pocket, and you can put your mp3 player in another pocket and music will just follow you around. It's great and loud for both the price ($95 at BestBuy.com) and its size. The OnTour takes 4 AAA batteries which last at least a few hours. AC adapter included.
JBL website Has the specs