One thing we hate about iPhone battery cases is the sheer bulk it adds to your phone. Incipio's new OffGrid case solves the bulk problem with a form factor that's thin and light ($70 - Mid December) and at the same time doubles your battery life. Link
Nomad designs a billfold with an integrated battery
You'll never forget your wallet again.
Apple releases an official battery pack for the iPhone 6/6S
Apple's option is one of the thinnest on the market.