Below I’ve compiled a graphic (for both Windows and Mac) showing my most commonly used shortcuts. I come from a landscape photography background but these shortcuts should be useful to those working in other genres as well.
Learning these shortcuts will save a lot of time. Whilst it might seem tedious at first it will soon repay the investment. The masking shortcuts in particular will allow you to paint or modify layer masks relatively efficiently.

Photoshop Shortcuts for Windows (Click to open full res image)

Photoshop Shortcuts for Mac (Click to open full res image)
Here are the shortcuts I have proposed laid out in text form should you wish to simply print the text versions. This are the Windows shortcuts, but to change them to Mac just replace Ctrl with Cmd.
General Shortcuts
Ctrl + X : Cut
Ctrl + C : Copy
Ctrl + V : Paste
Ctrl + S : Save
Ctrl + Z : Undo
Ctrl + Alt + Z : Step Backwards in History (repeatedly undo)
Movement Shortcuts
Ctrl + + : Zoom in
Ctrl + – : Zoom out
Space : Hold and mouse click to move with hand tool.
Masking Shortcuts
B: Brush
0–9: Set the brush opacity between 10% and 100%
[ : Smaller Brush
] : Larger Brush
Shift + [ : Softer Brush
Shift + ] : Harder Brush
Ctrl + I : Invert
X : Switch Foreground and Background Colours
Other Shortcuts:
Ctrl + T: Free Transform
Ctrl + J: Duplicate Layer
Thanks for sharing the shortcuts of photoshop, Alex. Now it would be much easier to work on photoshop and time savvy also!