System Indicator is a simple, clean, lightweight application sitting in the Mac OS X menubar. Its unobtrusive icons will inform you how much disk space, processor, and RAM is being used currently by your computer, while perfectly matching the black Mac OS X menu bar icon style.
The app is extremely light on resources itself, and will waste no screen space -- each of the indicators (CPU, RAM, disk space) can be off, an icon that changes color according to load, or an icon and a percentage indicator. Additionally, the CPU indicator can be configured to either show an average percentage for all cores, or a detailed view showing the precise usage of each of your computer's cores.
In other words, you will be able to configure the app to show as few or as many details as you want.
The menu bar icons also change color; to yellow if medium load is detected (above 65% for RAM and CPU, above 80% for disk) and to red if high load is detected (above 85% for RAM and CPU, above 90% for disk). You will therefore be able to gauge the load of your system at a glance, even if you disable showing percentages.
updated for MacBook Pro with Retina Display
System Indicator (com.houbysoft.system-indicator) is a Mac software application that has been discovered and submitted by users of Software.com.
Operating System: Mac
Default Install Path: /Applications/System Indicator.app
Minimum OS version: 10.6
Copyright: Copyright (c) 2011 Jan Dlabal. All rights reserved.