Mac OSX key sequences

The Mac has an additional 'modifier' key is often referred to as the Command key or the Apple key. It´s the one with an outline of an Apple and the little cloverleaf-like symbol on it.  There is no Windows equivalent, but your Windows key will map to it on your PC keyboard.  The Alt key in Windows and Linux becomes the Option key in Mac keyboard.

Apple-tab Switch (Cycle forward) open application windows.
Apple-shift-tab Switch apps in reverse order

Apple-option-D toggle dock - show and hide Dock.
Apple-option-esc - to kill an application

Apple-shift-3 Screen shot to a PNG file on the desktop
Apple-shift-4 Abstract user defined area screen shot to a PNG file

Apple-space Show Spotlight
Apple-Option-space Open Spolight form

Editor Action:

fn-Delete (Backspace) forward delete
fn-Left Arrow home
fn-Right Arrow end
fn-Up Arrow page up
fn-Down Arrow page down

Mouse Action:

control-Mouse Click - mouse right click

Display Action:

Apple-option-control-8 Turn on 'Inverse Mode' via accessbility.
Apple-option-8 Turn on 'Zoom Mode' via accessbility. 
Apple-option-plus Zoom In via accessbility.
Apple-option-minus Zoom Out via accessbility.