$49 for Mophie Juice Pack for iPhone 4/4S Air Battery Case for ($79.95 List Price). Three Colors Available.
Not only does the Juice Pack Air safeguard smartphones from drops and dings, it virtually doubles its functional life with a built-in rechargeable battery. The cases emulate the iPhone aesthetic and provide unencumbered access to all ports, buttons, and self-destruct switches. A handy switch toggles between standby and charging modes, and four LEDs on the bottom alert users to the battery pack's power level, which can be recharged with an included USB cord.
Slim, durable case with built-in rechargeable battery can virtually double the life of the iPhone 4/4S
Hard-shell case protects against drops, scrapes, and texting bears
Toggle between standby and charging modes
Four LEDs alert users to the battery pack's power level
Offers access to all buttons and ports
Included USB syncs to iTunes as it recharges
Case doubles as a speaker box, redirecting sound from the bottom of the iPhone 4 to the front
Rechargeable for over 500 cycles