Power users likely already know about Activity Monitor. This handy troubleshooting tool (/Applications/Utilities) can tell you which apps are running on your Mac, along with how much CPU and RAM they're gobbling up. But Activity Monitor packs a lesser-known yet equally useful feature: You can tweak its Dock icon to display live graphs charting what's going on with your computer.
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 app'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'll 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 most recently.
If you'd like to ensure that your preferred chart lives in your Dock full time, Control-click on the Dock icon again and choose Options -> Keep in Dock.