Keyboard Shortcuts for Photoshop and Photoshop Elements

Rectangular Marquee tool–M
Move tool–V
Lasso tool–L
Brush tool–B
Gradient tool–G
Clone Stamp tool–S
Crop tool–C
Polygonal Lasso tool–L
Magic Wand–W
Elliptical Marquee tool–M
Shape tools–U
Merge Down–Ctrl E (Mac: Cmd E)
Apply the last filter again–Ctrl F (Mac: Cmd F)
Copy–Ctrl C (Mac: Cmd C)
Paste–Ctrl V (Mac: Cmd V)
Create a new document–Ctrl N (Mac: Cmd N)
Open an existing document–Ctrl O (Mac: Cmd O)
Close a document–Ctrl W (Mac: Cmd W)
Select all–Ctrl A (Mac: Cmd A)
Make a merged copy on a single layer–Ctrl Alt Shift E (Mac: Cmd Opt Shift E)
Open Levels–Ctrl L (Mac: Cmd L)
Open Hue/Saturation–Ctrl U (Mac: Cmd U)
Turn to grayscale–Ctrl Shift U (Mac: Cmd Shift U)
Invert a selection–Ctrl Shift I (Mac: Cmd Shift I)
Open Preferences–Ctrl K (Mac: Cmd K)
Open the Image Size dialog box–Ctrl Alt I (Mac: Cmd Opt I)
Zoom in to your document–Ctrl + (Mac: Cmd +)
Zoom out of your document–Ctrl - (Mac: Cmd -)
Fit your document to the desktop Ctrl zero (Mac: Cmd zero)
Show all of a Transform outline that is partially hidden–Ctrl zero (Mac: Cmd zero)

Note: Some tools have the same keyboard shortcut. In that case, using the shortcut will toggle between those tools.