Audiophiles looking to boost the sound on their iPhone 4/4S should consider V-Moda's mobile amp. The VAMP utilizes a 150mW x 2 amplifier to convert digital audio to analog, yielding two high-fidelity listening modes: Pure and VQ. And as a bonus, the audio optical output device provides eight hours of playback from its built-in battery pack. The $650 price tag is steep, but try finding another headphone amp/DAC/battery pack combo of this caliber.
24. V-Moda VAMP