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Activity Monitor can (located in /Applications/Utilities/). Click on the System Memory tab at the bottom.
 
iStat makes a widget for the dashboard that will tell you memory usage and more.
 
You can use Activity Monitor, located in Applications > Utilites.

You can also install iStat menus, a free 3rd party application. They also have a dashboard widget version.

Finally, if you are so inclined, you can use the command line program "top".
 
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