It's called PowerSkin. It's a case that you put on your phone, and has an external battery on it. What is nice about it, is that it doesn't take up too much pocket space! It is pretty slim, and makes it easy to carry around, despite it having an extra battery on it. I bought my power skin about 2 months ago from Amazon, and it is probably the best accessory I have ever bought for it! When I first got it, it stopped working after about 2 weeks because the charging port would not connect properly to a cable. But when I contacted customer service, they responded the same day, and sent me a new one for free! They didn't have me pay for any shipping, or even send the old one back. It just got here within 2 days, and I'm using my new one now!
As for performance, it has significantly increased my battery life! I'm running on my original battery, which has gone down to only 6 hours of charge, (that's when I use it quite a bit) but has gone up to nearly 22 hours because of this case! I can't imagine how long this could go with the Galaxy Nexus battery!
And the best news of all, is that instead of being around $45 when it first released, I bought it for under $20! This is a great buy, & I recommend it to anyone! I would suggest going with Amazon, because that is where I found the best customer service!
I hope this helps!
Sent from The Blaze of The Doctor