It is estimated that you could save up to 8 days a year just by using shortcuts! So here are a list of useful shortcuts in Photoshop, with my favorites highlighted. And many of them work in Illustrator and InDesign as well. You can also define your own keyboard shortcuts. My favorite tip there is to define shortcuts to Tile images, and then to Consolidate all to Tabs.

Tools

Spacebar Hand/Move canvas
R Rotate View
D Default colors
X Switch Foreground and Background colors
V Move
M Marquee tools
L Lasso tools
W Quick Selection, Magic Wand
C Crop and Slice Tools
I Eyedropper, Color Sampler, Ruler, Note, Count
J Spot Healing Brush, Healing Brush, Patch, Red Eye
B Brush, Pencil, Color Replacement, Mixer Brush
S Clone Stamp, Pattern Stamp
E Eraser tools
G Gradient, Paint Bucket
O Dodge, Burn, Sponge
P Pen tools
T Type tools
A Path Selection, Direct Selection
U Rectangle, Rounded Rectangle, Ellipse, Polygon, Line, Custom Shape
To switch between all tools within groups, add the Shift key to the letters above
For example, to switch between rectangular and elliptical marquee hit Shift-M

Selecting

Draw Marquee from Center Alt-Marquee
Add to a Selection Shift
Subtract from a Selection Alt
Make Copy of Selection w/Move tool Alt-Drag Selection
Make Copy of Selection when not in Move tool Ctrl-Alt-Drag Selection
Move Selection (in 10-pixel Increments) Shift-Arrow Keys
Select all Opaque Pixels on Layer Ctrl-click on Layer Thumbnail (in Layers panel)
Restore Last Selection Ctrl-Shift-D
Move Marquee while drawing selection Hold Space while drawing marquee

Viewing

Fit on Screen Ctrl-0
100% View Level (Actual Pixels) Double-Click on Zoom Tool or Ctrl-Alt-0
Zoom in Ctrl-Space-Click or Ctrl-Plus(+)
Zoom out Alt-Space-Click or Ctrl-Minus(–)
Hide all tools and panels Tab
Rotate through full screen modes F
Scroll image left or right in window Ctrl-Shift-Page Up/Down
Jump/Zoom to part of Image Ctrl-drag in Navigator panel

Layer Shortcuts

Create new layer Ctrl-Shift-N
Duplicatge a layer CMD-J
Select non-contiguous layers Ctrl-Click layers
Select contiguous layers Click one layer, then Shift-Click another layer
Delete Layer Delete key (while in the Move tool)
View contents of layer mask Alt-Click layer mask icon
Temporarily turn off layer mask Shift-Click layer mask icon
Clone layer as you move it Alt-Drag
Find/Select layer containing object Right-Click on the object w/Move tool
Change layer opacity Number pad keys (w/Move tool selected)
Cycle down or up through blend modes Shift-Plus(+) or Minus(–)
Change to a specific blend mode (w/Move tool) Shift-Alt-letter (ie: N=Normal, M=Multiply. etc.)
Switch to layer below/above current layer Alt-[ or Alt-]

Type Shortcuts

Select all text on layer Double-Click on T thumbnail in Layers panel
Increase/Decrease size of selected text by 2 pts Ctrl-Shift->/<
Increase/Decrease size of selected text by 10 pts Ctrl-Shift-Alt->/<
Increase/Decrease kerning/tracking Alt-Right/Left Arrow
Align text left/center/right Ctrl-Shift-L/C/R

Painting

Hold down CTRL+Option and drag up/down for hardness, and left/right for size!

Fill selection with background color Ctrl-Backspace
Fill selection with foreground color Alt-Backspace
Display Fill dialog box Shift-Backspace
Sample as background color Alt-Click w/Eyedropper tool
To get Move tool While in any painting/editing tool-hold Ctrl
To get Eyedropper with Paint tools Alt
Change paint opacity (with Airbrush OFF) Number keys
Change paint opacity (with Airbrush ON) Shift-Number keys
Change Airbrush flow (with Airbrush ON) Number keys
Change Airbrush flow (with Airbrush OFF) Shift-Number keys
Cross-Hair Cursor Any painting/editing tool-turn Caps Lock on
Decrease/Increase Brush Size [ or ]
Decrease/Increase Hardness of Brush Shift-[ or Shift-]
Switch between preset Brushes < or >
Open Brushes pop-up panel Right-Click in Image window
Cycle down or up through blend modes Shift-Plus(+) or Minus(-)
Change to a specific blend mode Shift-Alt-letter (ie: N=Normal, M=Multiply, etc.)

Pen Tool Shortcuts

To get Direct Selection tool while using Pen Ctrl
Switch between Add-Anchor and Delete-Anchor Point tools Alt
Switch from Path Selection tool to Convert Point tool when pointer is over anchor point Ctrl-Alt
To Select a whole path w/Direct Selection tool Alt-click
Convert path to a selection Ctrl-click on path name (in Paths panel)

Other Shortcuts

Switch between open documents Ctrl-Tab
Undo or Redo operations beyond last one Ctrl-Alt-Z/Ctrl-Shift-Z
Reset Dialog Box Hold Alt, Cancel turns into Reset Button, Click it
Increase/Decrease value (in any option field) by 1 unit Up/Down Arrow
Increase/Decrease value (in any option field) by 10 units Shift-Up/Down Arrow
Replay last Transformation Ctrl-Shift-T
Move Crop Marquee while creating Hold Space while drawing