OS X: With Mountain Lion, Apple took away the ability to see your remaining battery time in the menu bar. Free app Battery Time Remaining brings it back, along with a few other goodies to keep your laptop alive.
Battery Time Remaining is a simple app that sits in your menu bar. It looks exactly like the default battery icon, but adds a battery life indicator next to it. Clicking on the icon brings you all the traditional options of the old battery icon, plus a few extra features under "Advanced Mode" — particularly the ability to customise battery notifications, so you know when it's reached a certain level. It's handy for those of us that are always whittling our battery down to its last few minutes.
Battery Time Remaining is a free download for OS X.