Activity Monitor (/Applications/Utilities) is a handy troubleshooting tool that can tell you which apps are running on your Mac, as well as how much CPU and RAM they’re gobbling up. But Activity Monitor has another clever way of helping you keep an eye on your system: In both Snow Leopard and Lion, you can make the application’s Dock icon display live graphs of your system’s status.
To change Activity Monitor from its default icon, first make sure the utility is running. Control-click (or right-click) on the Activity Monitor icon in the Dock, and select the Dock Icon option from the menu. (Alternatively, choose View > Dock Icon from the application’s menu bar.)
There, you can choose from five graph icons: CPU Usage, CPU History, Network Usage, Disk Activity and Memory Usage.
After you select one of them, you see a continuously updated chart in your Dock, graphing whichever stat you selected. If you quit and relaunch Activity Monitor, the software remembers the chart you selected.
If you’d like to ensure that your preferred chart lives in your Dock all the time, Control-click on the Dock icon again and choose Options > Keep In Dock.