There are fewer accessory purchases I have appreciated more than my portable USB charger. This one, with a truly massive capacity, is a mere $13.99 after applying its coupon on Amazon. It checks all the boxes you'd look for in an external battery of its kind, with support for speedy 2A charging output in one port and another available for simultaneous charging at conventional speeds. There is an indicator to let you know how much charge it has left, which is handier than you would think.
Oh yeah, it also boasts a 15,000 mAh capacity, which is just about as big as it gets in this category. If you're worried about the non-swappable battery in your new Galaxy S6, throw this in your backpack/briefcase/pocket and you'll be ready to recharge several times over without access to an outlet.
Already cheap at $19.99, using the coupon code GKN6HSSJ will save you an extra $6. The 39 Amazon reviews average out at 4.3 stars, which is more than solid. And at this price, I wouldn't be too outraged if it slightly underperformed.
Be aware that it is a little over 6 inches tall and about 0.8 inches thick, so this particular external battery is more about capacity than pocketability.