In many application for the Mac, you can have an instant dictionary at your disposal. Just hover over a word hold down the Control + Apple keys and then press the D key. You get an instant dictionary lookup of the word you are hovering on with a definition and thesaurus info. Even better, if you keep the Control + Apple keys pressed down without letting them up, you can move the cursor around and you'll get the dictionary lookup for every word that you move the mouse over.

Unfortunately, this dictionary trick doesn't work in every appliction, I really wish this worked for Firefox, but sadly it does not. But it does work in Mail, Word, and most other Cocca-based applications.