Some Android versions let the user take a screenshot natively, usually holding the Power and Volume Down buttons together. For all others, there's DroidAtScreen:
http://droid-at-screen.ribomation.com/
This application is invaluable if you want to display your Android screen on a computer and/or a projector.
On the Andbox, all you have to do is enable USB debug.
DroidAtScreen is a Java application that runs on the Linux box, where
it needs adb. Extract the .jar file and run it:
Linux:~$ java -jar droidAtScreen-1.0.2.jar &
Then configure DroidAtScreen telling it where to find the adb executable; type in its full path. You may have to click on Restart ADB and/or Reload Devices.