Many people are familiar with the Command+Shift+4 keyboard shortcut to access Mac OS X’s screenshot tool. This command lets you drag around an area of the screen to capture it as a PNG file on your desktop (use Command+Shift+3 to snap your entire display at once). If you press Space after invoking this command, you can capture any open window that you mouse over and click on.
If you add Control to this command (Command+Control+Shift+4), Mac OS X will capture your screenshot to the clipboard instead of creating a file on your desktop. This is ideal for pasting into image editors like Acorn and Pixelmator, and cutting down on desktop clutter.
In fact, the image editors are smart, and they’ll check your Mac’s clipboard for an image when creating a new file. If they see one, the new file dialog dimensions will default to those of the image on your clipboard.