I've just run across a useful little variant in how you can use Mac OS X's built-in dictionary service to see definitions of words in your documents. You undoubtedly know that you can Control-click or right-click any word and choose Look Up in Dictionary to display a little pop-up definition (some applications instead launch the full Dictionary application). And you may know that if you press Command-Control-D, the little dictionary pop-up appears for the word currently under the pointer. But if you press Command-Control-D and keep holding the Command and Control keys down, in either Tiger or Leopard, that little dictionary pop-up stays on screen and changes to define whatever word is under it as you move the pointer around. Try it yourself, or watch my brief screencast demonstrating the feature.
This feature works only in certain Mac OS X-native applications that support Apple's built-in dictionary, so it definitely won't work in Microsoft Word or Eudora, which use their own dictionaries, or in Firefox, which is barely a Macintosh app. However, it does work fine in the current versions of Mail, iChat, TextEdit, Safari, BBEdit, TextWrangler, iCab, NetNewsWire, Skype, Toast, and many others (thanks to folks on TidBITS Talk for testing).
Why might you want to use this trick? If you're editing a document that contains a number of unfamiliar words, it could be a fast way to learn their definitions. Or imagine that you're translating a document into another language, or learning a new language. I'd have been ecstatic to have a feature like this when I was learning Ancient Greek and Latin back in college, since looking up words in a separate dictionary slows down reading significantly.