I have a 2AA USB Charger that has multiple connects for Cell Phones. I bought it at Interstate Battery, and it works great for charging up my cell phone - and it fits in my pocket if I know I am going to be out all day and using my phone alot away from AC Power.

However, I just purchased one of these:

Opteka BP-SC4000 Ultra Thin Solar Powered High Capacity (4000mAh) Backup Battery and Charger for Cell Phones and Ipads.

This little gem has an internal battery that will charge my Cell phone up about three times over, and will charge my Ipad to 50% (about five hours heavy use).

It's about $25 on Amazon.

It also has a built in Solar Panel to charge the internal battery, but it really requires direct sunlight to charge in any kind of meaningful time. It's lightweight and I keep in in backpack.