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$20 OBO. Ultra Slim Extended Battery Case for iPhone 4/4S. Provides 1900 mAh of charge, which should double your iphone's life. It has lost a little capacity in the year that I have owned it, so that it now gives you about 1.7-1.8 times the normal life.
The product sells for $100 new and has been a lifesaver several times that I needed my phone but would otherwise have run out of batteries.
The product: http://www.mophie.com/shop/battery-cases-iphone-4s-4/juice-pack-plus-iphone-4s-4
