It a car charger.,
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This review is from: Shop4 Compact Micro Bullet USB In-Car Charger Adapter For Samsung Galaxy S4 SIV i9500 Mobile Phone - Blue (Electronics)
Not just for the Galaxy S4 as I have been using it for all kinds of mobile gadgets in the car. Nice and small so doesn't get in the way if you leave it plugged in. Does the job at a good price.