iPad (Jailbroken): One of the iPad's odd quirks is that you don't get any indication of how much time you have left to use it before the battery dies. Powerleft is a jailbreak tweak that adds that information, so you can know ahead of time if you can finish that movie before the battery dies.
Powerleft estimates the remaining usage time on your iPad just as your laptop does with its battery. Instead of getting a percentage, you get an actual time. The interesting thing about Powerleft is that you can also customise its display in a number of ways, including giving it an overlay window and adjusting where it sits. You can find Powerleft in the Big Boss Repository. Powerleft is currently only available for iPad, but an iPhone version is in the works.