If you Command-Option-click an item in the dock then all other windows (that do not belong to the item clicked) are hidden. This also works if the item clicked is an application that isn't currently running -- in this case the application is launched and then all other windows are hidden.
command click on an item in the dock: opens finder window to location of that app/file
Command-option click an item in the dock: hides all applications except for the one you clicked on
option click from one application to another: just like in os 9 and earlier, holding down option and swictching applications will hide the application that was active prior to you clicking on another app