iStat Menus

There are some utilities that no Mac should be without. And when they are free, you really have no excuse not to at least try it. iStat Menus is a great collection of menu bar items that monitor everything about your Mac. You can see CPU usage, temperature readings, power usage, drive space, and tons of other bits of data. CPU usage, for instance, can tell you which apps are hogging the most power. There is also a Dashboard widget version called iStat Pro.