From Mac Daily News...

With Mac OS X v10.2, you now have yet another option for capturing screenshots. This method is so often ignored that it may as well be a secret. We're here to let the secret out. The first two options were available in previous versions of Mac OS X (note: Command = Apple key):

1. Press Command-Shift-3 all at once to take a screenshot of your entire screen.
2. Press Command-Shift-4 all at once and Mac OS X presents you with crosshairs you can use to select whatever portion of your display you'd like to capture in a screenshot. Just select your desired area, let up on the mouse and whatever you've highlighted will be captured.

The new third option in Jag is very cool:
3. Type Command-Shift-4 and then press the spacebar once. The crosshairs will change into a camera icon. Hover the camera over any element on the screen you wish to capture, for example, the Dock, the menu bar, the desktop, or any open window. Then just click the mouse button to "snap" a screen shot. Or hit Command-. (Command-period) to cancel. With this option, you can entirely eliminate the desktop when you capture a screen shot of an individual window and get a clean outline without having to crop your screenshot in another application.

Cool, huh?