Q- Quick mask mode toggle = view selection as rubylith overlay, accepts paint tools
F- Cycles through Normal and 2 full-screen modes — very useful for previewing without clutter
Tab- Removes all palettes
Ctrl+; - Show/Hide Guides
Ctrl+' - Show/Hide Grid
Ctrl+R - Show/Hide Rulers

Layer

Ctrl+Shift+N- New Layer
Ctrl+E- Merge Down
Ctrl+J- New Layer from selection by Copy
Ctrl+Shift+J- New Layer from selection by Cut
Ctrl+G- Clipping Group with previous layer

Tools

V- moVe
M- Marquee (Shift+M, circular Marquee)
C Crop
W- Wand
B- Brush
E- Eraser
S- cloning Stamp

N- liNe
T- Type
K- fill bucKet
I- Idropper (eyedropper)
G- Gradient fill
P- Pen
O- Dodge / Burn / Saturate (Shift cycles through)
R- bluR / shaRpen / smeaR (Shift cycles through)
H- Hand
Z- Zoom

Color

D- Default colours (Foreground/Background to B/W)
X- Xchange (Swap) foreground/background colours)

Image tweaks

Ctrl+M- Gamma Curves
Ctrl+L- Levels
Ctrl+U- hUe / saturation / lightness
Ctrl+B- colour Balance

Random

Alt+Backspace- Fill with foreground colour
Alt+Shift+Backspace- Fill with foreground colour, preserving transparency
Shift+F5- Fill dialog box
[ ]- [Square brackets] Brush size up/down
ctrl+z-undo
ctrl+alt+z-undo further
Control + K - Removes the grey/blue box in the top left of your canvas.
Cntrol + Q - Closes the open document.