*3.x OS ONLY* Fine-grain control over the status bar battery. This will let you show percentage and/or the battery icon on any 3.x iDevice. The premium features (99c) add custom settings for low battery and charging, styles for the percentage, the ability to remove the percent symbol and tapping the battery to switch to percentage.
NOTE: Battery Control will NOT be updated to iOS4. There is simply too much that changed between the OS versions to make it viable at this point.