There are times where you absolutely need to charge up your iPhone 4 or 4S, but you're nowhere near a power outlet. In those situations, you might fall back upon an iPhone battery case like the trusty Mophie Juice Pack Air I carry with me, but what happens when that battery runs out? No problem, as long as you're carrying the Etón Mobius (US$80) rechargeable battery case with a solar panel and it's a sunny day.
The Mobius has a hefty capacity -- 1800 mAh -- and can be charged the time-honored way by plugging it into a USB port. But for those times where you're hiking, biking, camping or just not near a power outlet, you can place the case into the sunlight to charge the battery pack.
Since your iPhone (and any other mobile phone) can react badly to exposure to laying in the sun, Etón recommends taking the Mobius off of the iPhone before setting it in a sunny place for charging. Especially with the black Mobius case, you'll want to take care picking it up after it's been sitting in the sun for a while as it can get quite warm.