[stcredzero]

I have one of these, which you can instantly "recharge" at the corner store by buying 4 AA batteries.

http://www.tekkeon.com/products-tekcharge1580.html

I normally keep it filled with good quality rechargeable NiMH batteries (PowerEX) which hold more than enough juice to completely recharge an iPhone 4. 4 AA's from the store will give you about 2/3rds of a charge. I find it well designed. It also works as a travel charger.