The issue with most back-up batteries — we just don't want to carry them around. They're heavy, they're ugly, and they are such a bummer to use, we'd rather risk going into the red. However, these Power Mate Plus back-ups are compact, cute, and hold an amazingly long charge, so you won't find yourself in a battery bind ever again. Coming in a variety of prints and colors (as well as some super-special J. Crew-collab models), the charger is compatible with the iPhone 3GS, 4, and 4S and adds a 42% charge to your battery. Forget a Marc Jacobs invite — this is going to be the most important thing in our purse this Fashion Week!