You can display and monitor virtually all essential system activity directly from your Mac OS X menu bar using a great utility called iStat Menu:

CPU usage

Memory use

Disk capacity

Disk activity and I/O

Temperatures of CPU, battery, heatsinks, memory, hard drives, and more

Network activity and bandwidth usage

Calendar and time

Everything is really customizable so you can change the width, colors, and what and how things are displayed in the menu bar by tweaking settings in the iStat Menus Preference pane. For my uses, I show Disk IO, CPU activity, and bandwidth usage, but if you want to see everything going down on your Mac you can do that too.

Now here’s a little secret, iStat Menus 2.0 can still be downloaded for free from third parties.

Download the free version 2.0 from TuCows (supports Mac OS X 10.6.8 or lower)

Update: To clarify, the free version only supports Mac OS X 10.6 or lower, while iStat Menus 3 has full Mac OS X 10.7 Lion support and costs $16. Both were created by the same developers, the company just changed their name. Thanks to Valashtar for the clarification.

Here’s a look at the settings panel within System Preferences:

I much prefer this method to just showing CPU load in the Dock with Activity Monitor. One because it’s a smaller footprint, two it’s easier to spot more detail at the top of your screen, and three because you can pull down the menu options and see even more information about whatever it is you’re monitoring.

This is what the bandwidth monitor looks like when you click on the menubar item, it shows incoming and outgoing data, peak speeds, your IP, network location, and you can also access other network utilities from the pulldown menu.

This kind of detailed information is available for each aspect of your system that you are monitoring. Pulling down the CPU menu will show you the top processes, load average, uptime, and more. Disk Activity will display a chart of reads and writes for each drive, etc.

My only complaint is that the Memory monitor portion doesn’t show swap usage in the menubar, but considering you can usually feel and see swap usage (the spinning beach ball of death), it’s not that big of a deal. On the topic of RAM, if you are hitting virtual memory too often I suggest you check to see if your Mac needs a RAM upgrade, RAM is so cheap these days and makes a big difference in system performance so it’s hard to not justify the minimal expense to max it out.

All in all this is a great free app that is quickly disappearing from the web, so if you like to keep an eye on how your Mac handles it’s resources, grab iStats Menu 2.0 for free before it’s gone.