For a while now, the three-finger tap in OS X has been an easy way to look up the definition of a word. But in OS X 10.3.3 you can also use it to preview links in Apple's apps.
To get a link preview, just mouseover a link in Safari, Mail or Messages, then tap the trackpad with three fingers. You should get a popover that previews that link. Tap anywhere inside and it will open it in a new Safari tab. The three-finger tap only seems to work with the software listed above, but it's still pretty handy.
Mac tips: Preview links in Safari with a three-finger tap [Apple Insider]