You may not know this, but in OS X (especially Leopard), the spacebar is a very powerful key. Not only is it the key of choice in the Finder to view a file in Quicklook, it may also be your key of choice in dialog boxes. When a button is surrounded either graphite (as pictured below) or blue, just hit the spacebar and you activate that button, whereas if a button is graphite (as pictured below) or blue just hit the return (enter) key to activate that button.

Hopefully this little tip will help you improve your productivity.