Best external battery pack for on the go?

I am wondering what are the best external battery packs to use for my iPhone 4?

I don't need one specifically designed for the iPhone as those are usually more expensive. My budget is in between $25-$50. I want one small that could maybe fit in my pocket or a small compartment in my bag for easy access and one that charges fairly quickly. It preferably needs to charge my phone from 0 to full, but realistically half the battery should be fine. Any suggestions?