TYLT Energi 10K Power Block
My girlfriend uses the TYLT Energi 10k Portable Battery. It isn't very slim or lightweight, but it can actually charge three devices at once. The 10,400 mAh battery is big enough to charge both of our tablets, and a smartphone once. Or enough to charge her smartphone multiple times throughout the day, while traveling, or sitting on the couch.
This TYLT charger doesn't have the cables built in like most from the company, but it comes with a long 5ft micro-USB cord, and since it has three ports you can use any cable, from anyone, including an iPhone if you'd like to share the battery with a friend that uses Apple.