Any painting tool or shape tool + Alt (except impressionist brush)

Selects background color

+ Alt + click

Sets opacity, pressure, or exposure for painting mode

Any painting or editing tool + number keys (e.g., 0 = 100%, 1 = 10%, 4 then 5 in quick succession = 45%)

Cycles through blending modes

+

Fills selection/layer with foreground or background color

Alt + (Backspace or Delete), or Ctrl + (Backspace or Delete)

Displays Fill dialog box

+ Backspace

Locks transparent pixels on/off

/

Connects points with a straight line

Any painting tool + + click