The Magsafe is a fantastic innovation, but it has made it slightly easier to disconnect your Mac from the AC outlet by accident without realising it. UnPlugged is a little free utility that notifies you via Growl when the power cord is disconnected.
Now it's true that if you're running a portable Mac the screen is set to dim automatically on battery power by default, plus the power icon in the menu bar is set to change between a battery symbol to one with a AC plug. But if you happen to be running your Mac solely with an external display, it could be easy to miss the warning signs that you're now running on battery power.
UnPlugged runs in the background keeping out of your way till its called upon. It'll also display updates on charge status, notifying you on changes in power percentage intervals, which you can set with a slider from 1% to 50%.
Just like apps like DiskAlarm, UnPlugged doesn't do anything that Mac OS X doesn't do for you already, but it does make it more obvious and could be the alert you need to keep you from running out of juice when you thought you were plugged in.
If you need a simple power notifier, give UnPlugged a whirl and download it for free from the Mac App Store.