A modifier key alters, or "modifies" the way other keystrokes or mouse clicks are interpreted by Mac OS X. Modifier keys include: Command, Control, Option, Shift, Caps Lock, and the functio (fn) key (if your keyboard has a fn key), and various combinations of these. For example, pressing Control-Option-Command-Eject shuts down your computer. Leave the Option key out (Control-Command-Eject) to restart. Option-Command-Eject will put your computer to sleep.
Throw stuff away much?
Command-Delete to throw an item away
Shift-Command-Delete empties the trash.
Option-Shift-Command-Delete empties the trash without the annoying, "Are you sure you want to empty your trash?" confirmation dialog.
One of my favorites is the Option key, which sometimes has "alt" also on it. This is one of the keys I use the most often, system wide. If I want to copy a file or folder, I hold the option key down, click and drag the item, let go of the mouse and then let go of the option key. Writing that sentence took way longer than the actual task. Like most modifier keys, the rule of thumb is Modifier key first and last. That means that for example in the task I just described, copying a file or folder, the FIRST thing you do is hold the Option key, use your mouse as described above, and let go of the mouse FIRST before you release the Option key.
A few more:
Option-Command key combination while dragging makes an alias of the item
And who hasn't needed this one?
Option-Command-esc to Force Quit.
One of the blingiest is this one. While clicking and holding an icon in the dock, press your option key and you get more, that's right, options. Quit becomes Force Quit. Hide becomes Hide Others.
Hold Option down while starting iPhoto and iTunes and you can choose or create a new library. The next time you open either application, it will open to the last library you used.
Option-Start on iTunes looks like this:
Hold Option while clicking on File(on the Menu Bar) in any program and see what you get. For instance in iTunes, clicking on File then pressing Option, we see the command Display Duplicates change to Display Exact Duplicates. That would help me sort through my 243 Track Ones for sure. And in iPhoto, holding Option while clicking on Photos in the menu bar changes Delete from Album to Move to Trash (iPhoto Trash is a whole 'nuther blog). I could go on and on. But there are 3 hours of Family Guy playing in the other room, and my pop tart is ready.