I have found these keyboard shortcuts very useful when working in photoshop. Although this list is not a complete list of keyboard shortcuts, but is very helpful in common photoshop tasks.
Zoom In & Zoom Out
Control and "+" = Zoom in
Control and "-" = Zoom out
Control+0 = Fit to Screen
Alt+Control+0 = Actual Pixels
Control+Spacebar and clicking = Zooming in the part where we have clicked.
Alt+Spacebar and clicking = Zooming out in the part where we have clicked.
You can also zoom in or zoom out either by entering the value in the percentage box or by moving the slider to the left or right. Alternatively, you can also click on two triangles. As the picture showing, clicking on smaller triangles will result in zoom out action. On the other hand, clicking on bigger two triangles will result in zoom in.
You can zoom upto 1600% in photoshop.
Copy and Paste
Control+C = Copy.jpg">
Shift+Control+C = Copy mereged.
Control+V = Paste.jpg">
Shift+Control+V = Paste Into (paste into the selection)
Color Filling & Color Swaping
Alt+Backspace = Filling the selection with foreground color.
Ctrl+Backspace = Filling the selection with background color.
X = Swaping foreground and background
Control+' = Show gridlines.
If you press control + ' again, it will hide gridlines
Control + : = Show/Hide Guides
Control + R = Show/Hide Ruler
Control + J = Duplicating the layer
Control + T = Transform the layer
Control + ] = Bringing the layer forward to the previous layer
Control + [ = Sending the layer backward to the previous layer
Shift +Control + ] = Bringing the layer forward
Shift+Control+[ = Sending the layer backward
Shift + click on layers will result in grouping of those layers. Pressing Shift and clicking on the layers again will result in ungrouping
Control + E = Merge all the grouped layers
Control + F = Applying the last used filter
Alt + Control + F = Applying the last used filter with filter settings.
Shift+Control+F = Fading the last used filter
When move tool selected, pressing the alt key and any of the arrow key will duplicated the active layer.
When move tool selected, pressing any arrow key will move the layer one pixel depending upon which arrow key was pressed
When move tool selected, pressing shift+arrow key will move the active layer 10 pixels depending upon which arrow key was pressed.
When typing with type tool, pressing Enter from alphabetic key pad will result in creating a new paragraph. Pressing Enter from numeric keypad will result in end of typing.