If you have Tiger installed, hold your cursor over any word, anywhere. Then hold down [control]+[Apple]+[d]. After pressing those 3 keys, you’ll see a nifty little definition window pop up. You then can let up on those 3 keys and hold the cursor of any other word and the definition will change to the word that is under the cursor. (I am told that it doesn’t work in Firefox) I’d take and post a screenshot of what it looks like but I’m too lazy right now.