iStat Menus is a comprehensive set of system monitoring tools, providing a quick-access view of system activity in your menu bar.
Formerly a free preference pane, iStat Menus is now a paid, standalone app--but it offers a lot of functionality to justify its price, along with a clean interface and very low system overhead. You need to open iStat Menus when you're configuring it, but most of the time you'll only see the app in an array of highly configurable menu-bar icons. iStat Menus can help you track CPU activity, memory usage, disk usage, disk activity, network activity, sensors (such as fan speed and temp), date and time, and battery. In addition to at-a-glance readouts in the menu bar, you can also click on each item for quick access to related data, apps, and preferences--so for example, clicking on CPU will show you active processes without having to actually open Activity Monitor.
If you're new to iStat Menus, it's easy to miss the fact that you can customize the content and visuals of each menu-bar item extensively. For example with Sensors, you can choose to display over 20 individual voltage, temperature, and current readouts. Less extreme and perhaps more widely useful, the Battery preferences let you customize the menu-bar display for different battery states--so you can show, for example, a percentage while charging, but time remaining while draining. Most menus also give you the option for different graphic representations, with pie graphs, horizontal and vertical graphs, etc.
iStat Menus is incredibly useful for anyone who wants fast, comprehensive visibility into their system, but even casual users can find a lot here to like--such as the app's Date & Time, which improves on the Mac OS's built-in Date & Time display, letting you see a monthly calendar, a "fuzzy" clock, and the day and time in user-specified cities. iStat Menus' free trial is definitely worth checking out.