I've had this little powerhouse for a few months now and have been pretty happy with its performance. Unlike many backup batteries, it actually works and keeps on working, too!
This particular model is the Anker Astro - 5600 mAH of power in a 4.25 ounce package. Has a built in LED light, too. 5600 mAH will re-charge most smart phones 2-4 times - enough to keep your phone charged through a short term emergency.
Testing this unit out, I've used it as the sole power source for my everyday iPhone. I run my iPhone down to 40% per day on average--checking e-mail, listening to music and audio books, and some web surfing and YouTube. The Anker was able to keep it topped up for 3 days, no problemo.
Anker uses Samsung cells versus junky no-name batteries, and I've found that this unit does a perfect job of holding a charge. The Samsung batteries are rated for at least 500 charge cycles, which is pretty good.
Would I trust an Anker battery to work in extreme environments? Not really - these aren't ready for the Arctic, jumping out of air planes or fast roping out of helos into the Hindu Kush. But, they're sturdy enough to ride around in a bag and face anything you'd expect a consumer-grade piece of electronics to last through.
And, these little batteries are for more than just charging phones. Add a USB light like the Goal Zero Luna light and you've got instant task lighting. It's a very usable amount of light and nice and hands free to boot.
The Anker does come with its own built-in LED, too - it's about as bright as your run-of-the-mill key chain light and will last for hours and hours.
Cost was a mere $36.
Anker has a variety of designs in different size/battery capacity categories. Amazon carries all of them, and most are right around the $40 price point. Were I shopping for one right now, I'd probably go with the Astro2, which packs 8400 mAH into a sub 7-ounce package and comes with a $38 price tag. For a few more ounces and a few more dollars, you can step up to the Astro E4 with a big ol'13000 mAh capacity.