OS X Keyboard Shortcuts

Screen Capture

  ⌘ ⇧ 3 + Ctrl:           Screen to clipboard
  ⌘ ⇧ 4 + Drag + Ctrl:    Selection to Clipboard.
  ⌘ ⇧ 4 + Spacebar + Ctrl Window to clipboard [Camera]

  ⌘ ⇧ 3                   Screen to disk.
  ⌘ ⇧ 4 + Drag:           Selection to disk
  ⌘ ⇧ 4 + Spacebar:       Window to Disk [Camera]

Screen capture files are in .pdf format (on the desktop) to convert to .jpg, open and choose File, Export.
See also screencapture or Applications:Utilities:Grab

Startup and Shutdown:

  POWER         Sleep now
  POWER         Hold for 10 seconds (force shut down)  
  Ctrl Eject    Restart, Sleep, Shutdown dialog box
  ⌘ Ctrl Eject  Quit all applications and restart
  ⌘ ⌥ Eject     Sleep now
  ⌘ ⌥ + Esc     Force Quit
  ⌘ + .(period) Stop a process

Log out:

  ⌘ ⇧ Q         Log out (Apple Menu)
  ⌘ ⇧ ⌥ Q       Log out immediately (Apple Menu)

During Startup:

  Shift      Start up in Safe Boot mode - temporarily disable
              login items and non-essential kernel extensions.
  ⌘ V        Start up in Verbose mode.
  ⌘ S        Start up in Single-User mode (command line)
  X          Force OS X startup
  ⌘ ⌥ ⇧ ⌫   Bypass primary startup volume and seek a
              different startup volume (CD or external disk)
  C          Start up from a CD that has a system folder
  D          Start up from first partition
  N          Attempt to start up from a network server(NetBoot)
  R          Force PowerBook screen reset
  T          Start up in FireWire Target Disk mode
  ⌘ ⌥ P R   PRAM Reset - Hold down Cmd-Option-R with your left hand,
             then press POWER to start up and immediately press P with your right hand.
             See KB 2238  At the prompt you can  type: reset-nvram and/or reset-all 
  ⌘ ⌥ O F   Open Firmware
  mouse down Eject CD

Dock /Switch Applications

  ⌘ ⌥ D      Show/Hide Dock
  ⌘ Tab      Switch application
  ⌥ Dock     Switch application, hide previous one
  ⌘ Dock     Switch application, hide all others
  ⌘ ⇧ C      Show Colors palette in application
 Ctrl ⌥ ⌘ *  Toggle Monochrome
  ⌘ T        Show Font palette in apps toolbar button (top right)

Apple OS X Finder

  ⌥ + Drag   Copy
  ⌘ + Drag   Move
  ⌘ ⌥ + Drag Make alias in new location
  ⌘ D        Duplicate
  ⌘ L        Make Alias
  ⌘ R        Show original
  ⌘ O        Open
  ⌘ S        Save
  ⌘ ⇧ S      Save as
 
  ⌘ Q        Close Application
  ⌘ W        Close Window
  ⌘ ⌥ W      Close all Windows
 
  ⌘ I        Get Info
  ⌘ ⌥ I      Get Info (Attributes Inspector)
  ⌘ H        Hide Finder
  ⌘ ⌥ H      Hide Others
  ⌘ N        New Finder window
  ⌘ ⇧ N      New Folder
 
  ⌘ P        Print
  ⌘ T        Add to Favorites
  ⌘ Delete   Move to Trash
  ⌘ E        Eject
  ⌘ F        Find
  Tab        Highlight next item
  ⌘ + ↑      Move up one directory
  ⌘ + [      go back
  ⌘ + ]      go forward
  ⌘ ⇧ ⌫     Empty Trash (Finder Menu )
  ⌘ ⇧ ⌥ ⌫   Empty Trash without dialog

Text

  ⌥ ←       Move one word left
  ⌥ →       Move one word right
  ⌘ ←       Beginning of line
  ⌘ →       End of line
            Scroll one page up (cursor retains position)
            Scroll one page down (cursor retains position)
  ⌥         Move one page up  (reposition cursor)
  ⌥         Move one page down
            Scroll to beginning of document
            Scroll to end of document

  Add shift key ⇧ to the above to also SELECT the text.

  ⌥ ⌫       Delete word  
  ⌥ Del     Delete next word

⌘ is the Command or Apple key
⌥ is the Option or Alt key
⇧ is the Shift key
⌫ is the Backspace key

Related:

Developer.apple.com - Keyboard Shortcuts Quick Reference
docs.info.apple.com - Startup key combinations for Intel-based Macs