MightyMonitor is a cool little widget for users of the wireless Mighty Mouse or Apple Wireless Keyboard. It gives you a fuel gauge for the batteries in the aforementioned peripherals. I assume it is just reporting the same battery level information you can get in the Bluetooth tab of the Keyboard & Mouse Preference Pane, but it does keep it front and center, so you know when it's time for a new set of batteries.
Unfortunately, I have neither a wireless Mighty Mouse nor an Apple Wireless Keyboard on this machine, so the widget (as you can see on the right), isn't doing much. For for those of you who do, however, MightyMonitor is a free download from MacUpdate.