Exolife Exogear presently holding the lead, slightly ahead of the Mophie Juice Pack Air and the Mili PowerSpring. Just like these fine selections, the Aigo aiPower i616 claims to "virtually double" battery life, but it's got another trick up its sleeve: a hidden USB ribbon cable for both charging and data built right into the case. Should you want for a longer sync cable than it provides on its own, there's also a second microUSB port built into the left side, as well as a switch for the external 1500mAh battery and a handy button to display its current level of charge. Not bad for a pleasantly comparable 125.8 x 62.6 x 17.25mm, nor the $80 asking price.
If you're looking for something a little more svelte, however, the $60 InCase Snap Battery might be more to your liking -- it adds a 900mAh battery in a soft-touch frame that InCase claims is the slimmest around. We can't say it looks like it offers much protection, only covering the corners and rear of the phone, but fashion has long been a harsh mistress to those who obey her siren call.