Keeping a USB cable on hand to charge your phone or tablet on the fly is a wise move, but totally inconvenient. Your pockets are probably already packed full with earbuds, keys, everyday porta-tools, and tiny cannons.
Instead, get yourself the $25 ChargeCard, USB-charging cables for iPhones and Androids that fit seamlessly in your wallet, because it's the size of a credit card. Need some juice? Whip it out, fold back the flexible USB and 30-pin/Lightning/micro-USB ends, and charge your heart out.