From Ed at Small Dog Electronics:
Did you know that you can trigger Apple’s built-in dictionary in almost any system application, such as TextEdit, Safari, and Mail? This allows you to quickly get spelling and definitions for tens of thousands of words. If you are reading this newsletter in OS 10.4 Mail, hold down the command-control-d keys, all at the same time with your mouse over a word (the command key is the Apple key, next to the space-bar.)