One of Photoshop's key features is that almost everything in it can be controlled using keyboard shortcuts. This allows the user to stay in a creative thought pattern and quickly change tools in monotonous work that requires it.
Main Toolbar
Key Action Action Key
m Marquee Tool   Move Tool v
l Lasso Tool   Magic Wand Tool w
j Airbrush Tool   Paintbrush Tool b
s Rubber Stamp Tool   History Brush Tool y
e Eraser Tool   Pencil Tool n
r Blur Tool   Dodge Tool o
p Pen Tool   Type Tool t
u Measure Tool   Linear Gradient Tool g
k Paint Bucket Tool   Eyedropper Tool i
h Hand Tool   Zoom Tool z
d Default foreground and background colors. Switches between foreground and background colors. x
q Switches between standard mode or quick mask mode. Switches between screen modes. f
TAB Hides/Unhides open tools.    

File Menu
Key Action
CTRL + N New
CTRL + O Open
CTRL + ALT + O Open As
CTRL + W Close
CTRL + S Save
CTRL + SHIFT + S Save As
CTRL + ALT + S Save a Copy
CTRL + SHIFT + P Page Setup
CTRL + P Print
CTRL + K Preferences > General

Edit Menu
Key Action
CTRL + Z Undo
CTRL + X Cut
CTRL + C Copy
CTRL + SHIFT + C Copy Merged
CTRL + V Paste
CTRL + SHIFT + V Paste Into
CTRL + T Free Transform
CTRL + SHIFT + T Transform > Again

Image Menu
Key Action
CTRL + L Adjust > Levels
CTRL + SHIFT + L Adjust > Auto Levels
CTRL + ALT + SHIFT + L Adjust > Auto Contrast
CTRL + M Adjust > Curves
CTRL + B Adjust > Color Balance
CTRL + U Adjust > Hue/Saturation
CTRL + SHIFT + U Adjust > Desaturate
CTRL + I Adjust > Invert
CTRL + ALT + X Extract

Layer Menu
Key Action
CTRL + SHIFT + N New > Layer
CTRL + J New > Layer Via Copy
CTRL + SHIFT + J New > Layer Via Cut
CTRL + G Group with Previous
CTRL + SHIFT + G Ungroup
CTRL + SHIFT + ] Arrange > Bring to Front
CTRL + ] Arrange > Bring Forward
CTRL + [ Arrange > Send Backward
CTRL + SHIFT + [ Arrange > Send to Back
CTRL + E Merge Down
CTRL + SHIFT + E Merge Visible

Select Menu
Key Action
CTRL + A All
CTRL + D Deselect
CTRL + SHIFT + D Reselect
CTRL + SHIFT + I Inverse
CTRL + ALT + D Feather

Filter Menu
Key Action
CTRL + F Last Filter
CTRL + SHIFT + F Fade

View Menu
Key Action
CTRL + Y Preview > CMYK
CTRL + SHIFT + Y Gamut Warning
CTRL + + Zoom In
CTRL + - Zoom Out
CTRL + 0 Fit on Screen
CTRL + SHIFT + 0 Actual Pixels
CTRL + H Hide Edges
CTRL + SHIFT + H Hide Path
CTRL + R Show Rulers
CTRL + ; Hide Guides
CTRL + SHIFT + ; Snap To Guides
CTRL + ALT + ; Lock Guides
CTRL + " Show Grid
CTRL + SHIFT + " Snap To Grid

Help Menu
Key Action
F1 Contents

Other Shortcuts
Key Action
Page Up/Down Move view up/down 1 screen.
SHIFT + Page Up/Down Nudge view up/down.
CTRL + Page Up/Down Move view left/right 1 screen.
CTRL + SHIFT + Page Up/Down Nudge screen left/right.
CTRL + SHIFT + Z Previous History entry.
CTRL + ALT + Z Next History entry.
SHIFT + - Previous blending mode.
SHIFT + + Next blending mode