FiiO A1

The A1 comes with a trio of plastic clips in the box for you to slip it into and attach to your clothing or belt. Otherwise, it's a tiny metal box with a 3.5mm jack to plug your headphones into and another to attach a 3.5mm male-to-male cable (also included) that goes out to your phone.
The A1 charges over micro USB, and you should get around 13 hours of battery life. That's enough to get you through a long-haul flight, but it's more impressive still given the 160mAh capacity. And you can always hook it up to a regular battery pack or your laptop to keep you juiced if you're away from an outlet for a long time. Operating it is simple: You turn it on, and your music starts coming through.
On the A1 you've got volume controls which operate independently of those on your phone. The nature of this little beast is to add volume, and with the combination of the two level controls, you're able to make fine adjustments and get everything just so. Of course just adding volume is no good if the sound quality isn't preserved, but there's nothing to worry about there with the A1.
The other neat feature on the A1 is a selection of pre-set equalizer settings. There aren't many, and you don't have any control over tweaking them as you would in software on your phone. But they're much easier to get at than taking out your phone, unlocking it, and going into the equalizer. Just press the button and it cycles through so you instantly get a feel. The LED on the button flashes to correspond to the setting you're currently on.

Product Highlights
Tiny Size
Hardware Bass-Boost Settings
Four EQ Modes for Maximum Flexibility
Rechargeable Lithium-Ion Polymer Battery
3-Color Status LEDs
20 Hz to 20 kHz Frequency Response