One little feature I really liked about the iPhone 3GS over the regular iPhone 3G, was the battery life percentage displayed beside the battery icon at the top right of my iPhone.
When my wife showed me a new iPhone application she got called Battery Doctor. I mentioned that because I had the 3GS, I would have no need for it. Wow, was I ever wrong. First of all, never try and tell your wife that you know something she doesn't, second, this iPhone app does plenty more than I thought.
Some of the extra cool battery features you get for just 99 cents:
-Display of how much talk time you have left
-How long you can surf the internet
-How long you can be on U tube
-Audio playback time
-Playing 2D and 3D games
and the best of all.... time to recharge back to 100%
There are also some tips on how to maximize battery life, a log of recharging, and you can even select themes for the display.
This little app definitely kicks butt!!!