I bought my MiLi battery a few weeks ago off amazon for $25 or so, it's the perfect blend of size, price, and juice. It kept my Droid and my Moto Rokr bt headphones charged through two whole days of heavy use (GPS, 3G, BT, Pandora) and fits perfectly in my pocket. Nice, short USB cord (one of the winding ones) and mini and micro USB adapters that take up hardly any space at all. If you spend a lot of time away from a wall outlet, I definitely recommend this one.
Amazon.com: MiLi Power-Crystal 2000mAh Extended Rechargeable Battery Backup for Blackberry Bold 9700, 9000, Curve 8900, 8300, 8310, 8320, 8330, 8520, Storm 9530 9550 (Black): Electronics