Common Macintosh shortcut keys

cmd+N - Open a New Window
cmd+Shift+N - Create a New Folder
cmd+Down Arrow - Open a File or Folder
cmd+O (letter) or cmd+Down Arrow - Open a highlighted icon
Tab - Select the next alphabetical icon
shift+Tab - Select the previous alphabetical icon
Left Arrow (in icon view only) - Select an icon to the left
Right Arrow (In icon view only) - Select an icon to the right
Up Arrow - Select an icon (or file) above
Down Arrow - Select an icon (or file) below
cmd+D - Duplicate a selected item
option+Up Arrow - Open parent window
cmd+W - Close the Active Window
cmd+I - Get Info
cmd+del - Move selection to the trash
cmd+opt while inserting disk - Rebuild Disk Desktop

Common commands to use in applications

cmd+A - Select All items or text in active window
cmd+C - Copy the current selection to the clipboard
cmd+X - Cut the current selection, delete it from current window and put it on the clipboard
cmd+V - Paste clipboard contents at current cursor position
cmd+Z - Undo the last action
cmd+O - Open a file
cmd+P - Print the active window
cmd+S - Save the current file or document
cmd+W - Close active window
cmd+Q - Quit the active application