If you notice your iPhone running low on battery power, there is a quick way to see what apps are sucking up the juice. This can be useful if you need to squeeze a bit more time out of your phone before you get to recharge it.
Select Settings -> General -> Usage -> Battery Usage
This will display, in descending order, the apps using the most battery. If you want to close the battery hogs, double click the home button, scroll to the app to close, and swipe it up on the screen.
Another good trick for extending battery life is to lower the screen brightness. The screen brightness setting can be found by swiping up from the bottom of the home screen.