some shortcuts to remember. Original source: http://ss64.com/osx/syntax-keyboard.html

OS X Keyboard Shortcuts

    3                   Screen to jpeg file on desktop
    4 + Drag:           Selection to jpeg file on desktop
    4 + Spacebar:       Window (click camera) to jpeg file

    3 + Ctrl:           Screen to clipboard
    4 + Drag + Ctrl:    Selection to clipboard
    4 + Spacebar + Ctrl Window (Camera effect)

    D      Show/Hide Dock
   Tab      Switch applications (even while dragging a file)
   `        Switch between document Windows in current application
   Dock     Switch application, hide previous one
   Dock     Switch application, hide all others
 Ctrl   *  Toggle Monochrome Hi Visibility
   +drag will prevent dock icons from moving out of the way
             (useful when dragging items to the trash)

   + Drag   Explicit COPY (will copy even if destation is on same volume)
   + Drag   Explicit MOVE (will move even if destation is on different volume)
    + Drag Make alias in new location
   D        Duplicate
   L        Make Alias
   R        Show original
   O        Open item
   ↓        Open item
   S        Save
    S      Save as
   N        New Finder window
    N      New Folder
   Q        Quit Application
   W        Close Window
    W      Close all Windows
   I        Get Info
    I      Super Get Info (change whatever you click on)
    S      Show/Hide the Finder sidebar
    Y      Toggle Slideshow mode (show selected graphic files full-screen)
   P        Print
   T        Add to Favorites
   Delete   Move to Trash
   E        Eject
   F        Find
   ~        Cycle finder windows (including desktop)
  Tab        Highlight next item
   + double-click folder icon will open in new window
   + ↑      Move up one directory
   + [      go back
   + ]      go forward
    A     Open Applications Folder
    D     Open Desktop Folder
    U     Open Applications/Utilities/ Folder
    H     Open Home Folder
    K     Go to network
   K        Connect to server
    G     Go to folder... (type full path)
    ⌫     Empty Trash (Finder Menu )
     ⌫   Empty Trash without dialog

Finder-Hide App:
   H        Hide Application 
    H      Hide other applications

Hide all other apps*:
    + click on dock icon
    + click inside app window

Hide all other apps that are in front*:
   + click dock icon (of running app)
   + click inside app window

*Some older (pre-intel) apps may struggle with these

  Spacebar (or  Y)   Activate QuickLook on the selected file(s)
    Y               Full Screen QuickLook on the selected file(s)

   left/right arrow   Switch windows
   1                  Switch to 1st terminal window
   2,  3...          Switch to 2nd, 3rd window

   Spacebar           Activate/deactivate Spotlight
    Spacebar         Open Spotlight window
   Return /  Click   Open the top/next item
   ↓ / ↑              Move to next/previous category

   ←       Move one word left
   →       Move one word right
 Ctrl A     Beginning of Sentence (Home)
 Ctrl E     End of Sentence(End) 
   ←       Beginning of line
   →       End of line
   [       “  Smart quote (open/left)
    [     ” Smart quote (close/right)
   ]       ‘  Smart quote (open/left) Also Apostrophe: O‘Brien
    ]     ’ Smart quote (close/right)
            Scroll one page up (cursor retains position)
            Scroll one page down (cursor retains position)
           Move one page up  (reposition cursor)
           Move one page down
   ↑ or   Scroll to beginning of document
   ↓ or   Scroll to end of document

  Add shift key  to the above to also select the text.

   ⌫      Delete previous word  
   Del     Delete next word
 Ctrl D     Delete next character (Del)
    .     Show hidden files in open and save dialogs (Snow Leopard only)
    /     Open Application Help (Search)

   Scrollbar           Jump to exact scrollbar location
   Scrollbar arrows    Scroll page at a time (page up/down)
   Volume up/down      Open Sound preferences
  Control  Eject       Turn display off (brightness=0)
   Brightness up/down  Open Display preferences

  POWER         Sleep now
  POWER         Hold for 10 seconds (force shut down)  
  Ctrl Eject    Restart, Sleep, Shutdown dialog box
  Ctrl  Eject  Sleep the display now
    Eject    Sleep the computer now

   Ctrl Eject  Quit all applications and restart
    Ctrl Eject Shut down the computer now (Graceful Shutdown)
    Esc       Force Quit menu - selectively force quit applications
     Esc     Hold for three seconds to Force Quit the foremost application (Leopard/Snow Leopard)
   .(period)   Stop a process
   Ctrl+power  Immediate restart (dangerous, only use if the system is frozen)

    Q         Log out (Apple Menu)
     Q       Log out immediately (Apple Menu)

  Shift      Safe Boot mode - Press shift asap AFTER the startup chime 
   V        Start up in Verbose mode.
   S        Start up in Single-User mode (command line)
    X        Force OS X startup
     ⌫   Bypass primary startup volume (will seek DVD or external disk)
    C        Start up from a CD (if it 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
  Option     Select startup disk 
    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
   E        Eject CD (press during/after startup chime)
  mouse      Eject CD (hold mouse button down during startup)

 is the Command or Apple key
 is the Option or Alt key
 is the Shift key
⌫ is the Backspace key

Toggle hidden files

defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder