COMMAND+OPTION:
  • Force open a file by dragging on to app icons
  • Create aliases by dragging one file from one place to another.
  • Customize toolbar by clicking the tube button on the top right of the window
  • Close all windows in a certain app.
  • Drag item out of dock to make alias in finder
  • Not that this matters, but, while option clicking a folder in Finder will open it in a new window, this key combo will open the folder in a new window and close the other window. Kinda pointless right?! This also works with sidebar items.
  • Click on a different application to hide all but the one you clicked on. This only works when the application you want for it to work on is not in focus. The keyboard equivalent of this is OPTION+COMMAND+H

OPTION:
  • Choose different iTunes libraries when opening iTunes
  • Choose different iPhoto libraries when opening iPhoto
  • Click on checkmark next to users name in System Preferences - Sharing - Apple Remote Acces - Access Privileges... to activate all options at once.  This also works in many other applications where there is a list of check boxes, ie when checking or unchecking songs on an iTunes playlist.
  • Open new inspector by clicking on a new tab
  • Copy instead of move files to new folders.
  • Click on a new app to hide all the others
  • Download linked file in Safari
  • ^ click or right click in Finder for altered plug ins
  • Menu items will change while holding option. 
  • Drag items out of the dock to created copy in finder
  • If enabled in Terminal window settings in emulation, click to position cursor at that point
  • Delete a file in a playlist in iTunes.
  • Create smart groups in certain apps like iTunes, Address Book, iPhoto, etc.
  • Click on the X on a safari tab while having others open to close all the others.
  • Move cursor over a dashboard widget to find the X without having to click the plus on the bottom left of the screen.
  • On laptops, click on the function keys, (if enabled for hardware features), to open their related preferences.
  • While dragging a file into iTunes, hold OPTION to make it so, (if copy to iTunes library is enabled), it will not copy it. This is good to change labels on videos that you do not want to wait for to copy into iTunes.
  • Click on a file, (not a folder,) in Finder and it will close that Finder window and open the file. This is good of you don’t care about the Finder window after you open the file.
  • Certain system wide functions, like logout, restart, shutdown, etc. And empty trash, though that isn’t system wide. Will no longer show the are you sure window box while holding OPTION.
  • Change the size of the dock in increments rather than pixels
  • Click the minimize (-) button to minimize all windows of an application.
  • FOR AIRPORT EXTREME 802.11 N ONLY: In airport utility on the radio mode dropdown menu, using option and clicking on the dropdown allows you to choose many more radio modes.
  • Address Book: highlights the groups a person is associated with

COMMAND:
  • Click on dock items to show in finder
  • TAB to cycle through opened apps
  • SHIFT-TAB to cycle in reversed direction
    • ...while holding the command key down:
      • repeat hitting TAB to cycle one at a time
      • hold the TAB key down to scroll though the apps quickly
      • ` to reverse direction; hold it down to scroll quickly (Note: you must hit COMMAND-TAB first and continue to hold down COMMAND before pressing `)
      • type "q" to quit the app
      • type "h" to hide the app
    • Start Expose (F9 or F10), then command-tab to cycle between apps
  • Click on spotlight results to show in finder
  • Down arrow or Up arrow to jump file categories in Spotlight menu
  • Down arrow on item(s) {file(s)/folder(s)/Disk(s)} in the finder to open the item
  • Drag items out of the dock to created copy in finder
  • Open link in new tabs in Safari, including forward and back buttons.
  • Open link in a new window in Safari, when tabs are disabled.
  • Open finder folders  in a new window. This works with sidebar items as well.
  • Drag toolbar items to remove or reorder in toolbars.
  • Drag menu bar items to reorder them or remove them completely.
  • Click the tube button (top right) and the icons in the toolbar will cycle through size and text settings...
  • Click on the name of a window (folder/app) to get the full path (Mac Os9 old syle behavior :)
  • Drag selected Finder items from one disk or volume to another and hold down Command to "move" the items, i.e. copy items to new location, then delete originals.
  • Push power button for 13 seconds to unveil a secret easter egg
    • What does it do?  I don't have my iBook with me, and the number 13 seems kinda fishy :-)
    • This is part of the process of upgrading the Firmware. It will flash the power indicator and sound a tone, but I don't think it will do much else unless you are updating the firmware.

COMMAND+SHIFT:
  • Click on a link, in Safari, to open in a new background window.
  • right arrow to cycle to next tab in safari
  • left arrow to cycle to previous tab in safari

SHIFT:
  • Hold shift while clicking on the minimize button (yellow minus) and the window will minimize really slow to show off the effect. This works in reverse as well. Hold shift when restoring from the Dock, and it will restore slowly as well. You may also hold SHIFT while opening expose and dashboard for a very slow transition.
  • In list view, select an item, hold shift and click another item. All items between the first and second clicks will be selected.
  • Click on a preference pane to open it with a slowed down fade.

Shutdown, Restart, Log Out, and Sleep
  • ^+OPTION+COMMAND+EJECT - hold for 1 - 2 seconds to shutdown
  • ^+COMMAND+EJECT - hold for 1 - 2 seconds to restart
  • OPTION+COMMAND+EJECT - hold for 1 - 2 seconds to sleep (this is very helpful with desktop computer)
  • OPTION+COMMAND+SHIFT+Q - Logout without reminder saying are you sure.
  • ^+EJECT - display the restart/sleep/shutdown/cancel window


More Mac OS X Keyboard Shortcut lists:

safe mode