Quick keys

Screen Grab             Command + Shift + 4

Save File                  Command + S

Quit Application      Command + Q 

Create New File      Command + N  

Open Existing Image Command + O 

Enlarge/Magnify   Command +  +(plus key) 

Shrink   Command +  - (minus key)

Guidelines on/off    Command + ; (semi-colon key)

Scale or Rotate Command + T
Hue/Saturation Command + U 

Select All Command + A 

Undo Command + Z 

Drop Selection Command + D 

Levels Command + L 

Paste Command + V 

Copy Command + C 

Cut Command + X 

Rulers Command + R

Move Image Hold Space Bar

 Scale Brushes Bracket Keys

Presentation Mode F (click twice) 

Invert selection Command + Shift + I 

Duplicate Selection Command + Option + Drag

Switches Color Swatches X 

Defaults Color Swatches Blk/Wht D 

Invert Image Command + I