The Splash Intense 5400mAh external battery pack is already quite a popular accessory among HTC EVO 4G users (G&E's review here), but since choice is rarely a bad thing, let's take a quick look at Amzer's 3500mAh solar charger ($67). It's more expensive and has less capacity than the Splash Intense, but it can be charged by the sun, has higher amperage, is smaller and lighter, and can be turned off when not in use to minimize battery drain.
The charger is made pretty well and is small/light enough to slip into your bag unnoticed. There's something decidedly iPhone-ish about the look of it, I agree, but if the resemblance is the only thing stopping you from buying it, then that's your own personal anti-Apple issue.
Its design is simple and straightforward: power and USB ports at the top, on/off switch on one side, power indicator button on the other side, and corresponding power LEDs and of course the solar panel on the front.
There's also an LED built into the perimeter of the power port to show charging status (red or green). In a brighter environment, the glow is much less intense and more "contained" than what you see above.
The size of the solar panels should clue you in to the fact that you won't get a full charge from sunlight very quickly, but what's nice is that it doesn't need direct sun exposure to start charging. Here it is by the window in an air-conditioned car (notice the red charging LED):
It's probably not getting much charge like this, but since anything is better than nothing, you could drop it on the passenger seat when you're driving to work (or on the dashboard or back seat if you're carpooling), take it out when you're waiting for the bus/train, or leave it outside while you're camping and least get something. Even though charging via USB is considerably faster, it's always a good idea to use a natural energy source like the sun when possible.
My EVO is still being powered by the slim 1750mAh battery, which the Amzer charger took from 30% to 100% in about 3.25 hours. It used 1 of 4 power LED lights in the process, so it should be able to fully charge the EVO's stock 1500mAh battery several times before needing a USB port or some time to recharge in the sun.
The Splash Intense may be a better buy ($47 on Amazon) but solar power and a way to turn the battery off for an extra $20 isn't too bad. The Amzer solar charger is currently available for $67 on Amazon.