Ctrl + Esc

Opens the Main Menu.

Super + R

Displays the Run Application dialog.

Print Screen

Takes a screenshot.  

Alt + Print Screen

Takes a screenshot of the window that has focus.  

Ctrl + Alt + right arrow

Switches to the workspace to the right of the current workspace. 

Ctrl + Alt + left arrow

Switches to the workspace to the left of the current workspace. 

Ctrl + Alt + up arrow

Switches to the workspace above the current workspace. 

Ctrl + Alt + down arrow

Switches to the workspace below the current workspace. 

Ctrl + Alt + d

Minimizes all windows, and gives focus to the desktop. 

F1

Starts the online help browser, and displays appropriate online help.