Here are the most essential keyboard shortcuts for Photoshop 6.
These are not a complete list of shortcuts, but if you memorize these shortcuts it will really speed up your work!
Remember that all system shortcuts work in Photoshop. Ctrl + C is copy, Ctrl + V is Paste, Ctrl + X is Cut and so on.

Keyboard What it does
F7 Toggles Layers/Channels/Paths palette
F8 Toggles info palette
Ctrl + Shift + I Create the inverse of the current selection marquee
Ctrl + spacebar Toggles to Zoom In tool
Ctrl + Alt + spacebar Toggles to Zoom Out tool
Ctrl + Plus key Zooms in
Ctrl + Minus key Zooms out
Spacebar Toggles to Hand tool
M* Marquee tool
L* Lasso tool
B* Paintbrush tool
Right-click Displays Context menu
Ctrl + T Displays Transform box around current selection
Ctrl + L Levels command
Ctrl + U Hue/Saturation command
Ctrl + B Color Balance command
Keypad numbers Increase or decrease opacity by 10% (if you press two numbers in succession, such as 3,5, you get 35% opacity).
V Move tool
Ctrl held with a selection tool Move tool
Ctrl + R Rulers
Ctrl + H Hide screen elements, usually the marquee selections
Ctrl + D Deselect active selection
Ctrl + A Select all
* By holding Shift, and pressing these keys numerous times, you toggle through the hidden tools.