If you are doing a website about Leopard tips :p, or anything that requires tons of screenshots, here's a few goodies for you:

The basic screen shots keys are :

Shoot all the screen, creates a pic on the desktop:
Command () + Shift +3

Shoot a selection of the screen, creates a pic on the desktop:
Command () + Shift + 4

Shoot a single window:
Command () + Shift + 4 then press Space Bar : your mouse cursor changes into a camera and you can choose the window you want to shoot.

To copy the screenshot into the Clipboard:
Either add the Control key to the previous combo, or press Control while you release the mouse button (and take the actual shot).

Moving the selection:
While in selection mode, (command () + shift + 4), you can move your selection after or while drawing it by holding Space Bar. Note : you can lock one axis by pressing shift at the same time.

Goodies:
Once you've started drawing your selection, you can lock the height or the width by holding the shift key, and you can change the center of the selection by holding the Option (alt) key.