Many people know that if you press Command-Shift-4, then press the space bar, you can make a screenshot of a window without the need to painfully select it using the marquee. The mouse pointer turns into a camera; point and click at the window you want to capture.
But it's even better than that: Even if the window is bigger (wider or taller) than the actual screen, the full content (including the stuff which is outside) is grabbed.
One minor issue: In most apps it is not possible to move a window out of the top of the screen, which would enable you to resize it bigger than the actual screen. Reason: The window is blocked by the menu bar. So if you try it with your secondary screen (in dual head mode), it works.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060629235932385