Ironically, because the Apple MacBook Air has a longer battery life than earlier laptops, there's more need to watch how much juice is still in the tank.
In the old days you just knew you didn't have much time left. With ten hours or so of computing from a single charge, it's easier to lose track.
Battery Diag from the Mac App Store is a the best tool I've seen for monitoring the battery. It's free and pretty, with a design that echoes the iOS 7 design found on iPads and iPhones.
The app runs in the menu bar, so you can get at it quickly, it sips resources and stays out-of-the-way until needed.
Click on the menu bar icon to get a report on the amount of power left both as a percentage and as a time estimate. There's also an indicator showing the state of battery health and number of charge cycles. Further information, including battery temperature and power usage is hidden behind an I icon.
The clever bit is that if you're running out of juice, you can tinker with your open apps and usage to trim the power drain and extend the time remaining.