Stalion Strength Replacement Battery for iPhone 3G is a pick if you are still rocking the iPhone 3G.
With a battery capacity of 1050mAh, this Lithium Ion replacement battery for iPhone 3 has voltage of 3.7V and duration of 4.51Wh, and it is CE and ROHS certified. The tools needed for the replacement do not come with this kit and need to be bough separately.
Make sure to view the instructions on how to replace the battery safely. It comes with a warranty covering 12 months / 300 charge cycles.