Man key commands are the best thing in the world, here's a few off the top of my head:
ctrl+J = Duplicate layer
ctrl+C = Copy
ctrl+V = Paste
ctrl+X = Cut
ctrl+N = New Document
ctrl+shift+N = New Layer
ctrl+T = Transform(change the size of currently selected layer)
ctrl+U = Hue/Saturation
ctrl+S = Save
ctrl+shift+S = Save As
ctrl+; = Hide/Show Rulers that were pulled onto project
ctrl+r = show rulers
ctrl+Y = Change setting from CMYK to RGB and visa-versa
ctrl+A = Select All
ctrl+D = Deselect
ctrl+Z = Undo(press again to redo)
ctrl+alt+Z = Undo again
ctrl+O = Open
ctrl+shift+O = Browse
ctrl+alt+O = Open As