Photoshop - Shortcut Keys 

All of the tools on the Toolbar can be accessed using a keyboard key and its worth learning at least some of them to speed up your work. The letters for selecting from the Toolbar can be seen by hovering the mouse pointer over the tool.

In addition, many of the tools have several variations (eg Regular, Polygonal and Magnetic Lasso) which means you may have to right click the icon before selecting. By pressing the [Shift] key and the appropriate letter, you can cycle around the variations, or easier still, if you set the correct option, then simply pressing the letter repeatedly will cycle around the tool variations.  Find the option on Edit, Preferences, General, (or Ctrl K),  and un-check Use Shift Key for Tool Switch. 

Other keys & combinations are: 

Spacebar  Scrolling hand
Ctrl  Spacebar Zoom in
Alt  Spacebar Zoom out
Tab Hide or reveal palettes
Alt Zoom in or out with a mouse wheel.
D Set foreground to black and background to white (Default) Opposite for mask
Swap foreground and background colours
1 through 0 Number keys change layer opacity 1 = 10% to 0 = 100%   (not with all tools!)
Ctrl  Temporary Move tool.
Ctrl  H Hide marquee
Ctrl D Remove marquee
Ctrl E Merge layer down Shift
Ctrl E Merge all layers
Ctrl J  Copy selection to new layer
Ctrl T Transform selection
Backspace Del Clear
Ctrl Backspace Fill with background colour
Alt Backspace Fill with foreground colour
Shift Backspace Show Fill Dialogue Panel
Reduce brush size works on the fly
Increase brush size works on the fly
Shift  [  Reduce brush hardness (5 steps  100%, 80%, 60%, 40%, 20%) works on the fly
Shift  ]   Increase brush hardness (5 steps - 20%, 40%, 60%, 80% 100%) works on the fly
The four shortcuts above work with the Brush, the Stamp, the Eraser, the Blur Tool and the Dodge Tool.
F1 Help
F4 Paste clipboard to new layer
F5 Brushes - [Esc] to remove
F12 Revert to original image
[Ctrl] [0] zero Fit image on screen
Double-click hand icon Fit image on screen
Channels
[Ctrl][1] Red Channel Use these when looking for
the most contrasty channel
when making a selection
with the Magic Wand
[Ctrl][2] Green Channel
[Ctrl][3] Blue Channel 
[Shift][Ctrl][~] RGB Channels
[Ctrl] Click saved selection channel thumbnail to load selection
[Ctrl][F6] Cycle round open images only if more than one obviously
Masks
Double-click Layer Mask Options Panel
[Alt] Click Load mask to screen - in place of image.
[Ctrl] Click  Load mask as selection
[Shift][Alt] Click Load mask as red transparency with image