SlimBatteryMonitor is a replacement power gauge for Apple's Mac OS X that tracks both laptop batteries and many UPS batteries. Multiple-battery systems (e.g. older powerbooks) are supported as well. A graphical icon shows the power remaining, and can be accompanied by a text description (battery charge in percent, or time remaining). Colours can indicate whether the system is fully charged, charging or on battery.

Requirements:
OS X 10.5 - 10.6
Universal Binary

Also available is:
Version 1.4 for OS X 10.3 - 10.4