External battery packs are pretty simple. Unless you're specifically looking for something small or stylish, you generally want the biggest capacity for the cheapest price. One Amazon listing is delivering just that today: a 10,400mAh battery for $19.99. This Lumsing model should charge most Android smartphones at least three times over, more if you've got a smaller or older device. It doesn't look half bad, either - the description says it's designed to look like a harmonica, but I wouldn't recommend blowing into the holes.
Lumsing is a bit of a no-name when compared to Amazon regulars like Anker, but this model reportedly uses Samsung battery cells and comes with a one year warranty. The "Harmonica" battery has a list price of $60.00, but like most of the batteries with listing titles even longer than the stories on Android Police, that doesn't mean much. Like most of the competition, it includes two USB ports, one with 1 amp and the other with 2.1 amps. Recharging is done with any MicroUSB cord, though you'll need a 1.5 amp adapter (not included) to do so at full speed. The unit has four small LED lights to indicate its own charge level.
Reviews are positive, and Amazon Prime subscribers in the US are eligible for free shipping. The battery comes in black, white, and "champagne gold" colors - I'd go with the white, just so that the inevitable bumps and scratches it gets from riding in your carry-on won't show up as much.