I know your Mac is really cool, and you know it's really cool, but do all of your friends (especially those using Windows)? One of the things I really enjoy is
showing off some of my Mac's more interesting tricks, a job made easier by this cool list
at Silver Mac. They describe one of my favorite tricks right off the bat: If you highlight at word in any cocoa app (like Safari) and hit Command-Control-D, a small window will appear with the dictionary definition and part of speech of that particular word. Move the cursor from word to word, and see their respective definitions.
The article also describes how to invert your screen's colors, create a quick text clipping and more. Check it out, and really "wow" your friends.