Battery Mate is a quick and simple free battery app with an extremely customizable widget showing your exact battery level at any time.
Another feature is a notification icon for your status bar which can show your battery level or battery temperature in either Celcius or Fahrenheit.
The app itself shows the battery level, battery temperature and the length of time it has taken for the battery to lose the last 1%. It also provides links to see advanced battery information and for you to see which apps have been using your battery up.
An extremely customizable widget (in 1x1, 2x1, 1x2 and 2x2 sized versions), with options for text and background colors, backgrounds including a a moving ring, a pie chart style view, horizontal and vertically moving rectangles. To add the widget to your homescreen select it from Menu > Add > Widgets > Battery Mate.
You can also set up an alarm so that the phone will either vibrate or emit a sound when the percentage drops below the level of your choice.