These are the Photoshop shortcuts that have improved my efficiency when designing websites the most. Some are basic; some are a little more obscure. I thought I would share them.

Note that these are focused on web designers and may not be the most important shortcuts for people that actually use Photoshop to, you know, work on photos. Oh, and these are Windows shortcuts, though other platforms should be similar.

Shift-Ctrl + C	Copies to the clipboard from all layers, not just the active one. Beyond useful for creating the background images that make up your layout. No more flattening your design to extract images, accidentally saving, and then losing all your hard work. ;)
Shift-Ctrl-Alt + N	Creates a new layer immediately with no dialog. Because who needs dialogs?
Ctrl + New Layer	Clicking the new layer button in the layers box while holding Ctrl will make the new layer get created underneath the current active layer, instead of on top. I haven't found a way to combine this shortcut and the one above yet. If you know a way, post it below!
D	Resets the foreground/background color to the default (black/white normally). I find myself tapping D habitually, even when the colors are already defaulted. Gotta work on that.
X	Swaps the foreground and background colors.
Shift-Ctrl Z	Cycles backwards through the history.
[ and ]	These two keys are very useful. When any brush-type tool is selected, they increase/decrease the size of the brush. Holding Shift while pressing them increases/decreases the hardness of the brush. Never use the brush dialog again! They have many uses that go beyond this too, including moving around in the layers box with Alt and Ctrl.