Amazon is currently having a sale on the Motorola 3,000mAh Power Pack for $8.99. This normally goes for $50, folks. So if you're in need of an external charger for your phone you could certainly do worse.
EDIT. Looks like Amazon's put the price up to $14.99, which umataro42 noticed and I confirmed seconds ago. Still, $14.99 isn't too bad considering it's regular price.
For $10.89 you can snag the Motorola Power Pack Slim 2000 from Amazon though. (Thanks for the link umataro42!) For those wondering, those slim ones from Motorola are basically the size of your phone, but obviously slimmer and they work well. One of my friends has one, forget the exact model though, and it's very slick looking. I sometimes forget it's a charger and think they're carrying around a second phone.
Although eBay has an additional one you might want to consider. A 2-pack of the same external charger for $14.99.
As a fan of Motorola's products I'm just gonna go ahead and recommend these on principle, especially at those price points. Short of when there are sales or promotions it's not uncommon to pay $40+ for external chargers.
For the record, something similar from Anker (specifically a 3,200mAh lipstick sized portable charger) goes for $16.99 on Amazon.
Whatever you get, I can heartily recommend external battery packs/chargers. At worst it's $20 or less and you throw it in your glove box or center console and use it in an emergency or when you need a quick charge while on the go. It's what I do with mine and it has saved my ass more times than I can count in the relatively short time I've had it (ignoring the fact that time to me is all but an illusion, lunch time doubly so).