Mac OS X Leopard only: Reader Ben points out that there's a lot more to Mac OS X's built-in Dictionary than definitions. He writes in:
I just noticed that in Dictionary.app (at least in Leopard), under to "Go" menu, there is an option labelled "Front/Back Matter." Clicking this brings up a whole slew of useful reference material, including a language guide (complete with a list of clichés to avoid), a list of the chemical elements, and the text of the Constitution. Next time I'm on a place, I think I'm going to brush up on the Bill of Rights.
It's true: the Ready Reference area of Dictionary.app's Front/Back Matter is a treasure trove of good stuff for writers or anyone who wants to see if they can name the capital of all fifty states in the U.S. Thanks, Ben!