All these keyboard shortcuts are composed of a combination of several keys whose name or appearance may differ depending on the age of your machine.

- Command + N: Open a new Finder window

- Command + Shift + N: Create a new directory in the Finder

- Command + I: Displaying the information of an element

- Command + D: Duplicate the selected elements.

Note that when you are saving a document from an application, this shortcut allows you to choose the Desktop as the saving destination.

- Command + T: Open a new tab in the Finder, a web browser

- Command + Q: Exit (Close) an application

- Command + Tab: Switch from one open application to another

- Command + H: Hide the active application window

- Command + Option + H: Hides all active windows except the one in the foreground

- Command + M: Minimize the active window in the dock

- Command + Option + M: Minimize all windows of the active application, except the one in the foreground

- Command +, (comma) : Display the parameters of the active application

- Command + Option + D: Show or hide the Dock

- Command + Delete: Place one or more elements directly in the recycle bin

- Shift + Command + Delete: Empty the contents of the recycle bin

- Control + lateral directional arrows: Switch from one desktop to another

- Space bar or Command + Y: display a preview of the selected file

- Command + Space: Display the Spotlight search field

Option + Command + Esc: Force a blocked application to close 

Source :01net.com