http://www.binaryfortress.com/displayfusion/

DisplayFusion is a fantastic (and free) new application that can make your dual monitor (or triple monitor or more) life much, much easier! From allowing you to use a different wallpaper on each monitor, to integrating with Flickr for image searching, to providing hotkeys for managing your application windows.
Here are just a few of the things you can do with DisplayFusion:
  • Set a different desktop background on each monitor (either a picture or solid colour)
  • Set a desktop background that spans all monitors (either a picture or colour)
  • Easily manage application windows with HotKeys:
    • Move windows to the next monitor
    • Move windows to the next monitor and maximize them
    • Move windows to centre of the screen
    • Move windows to centre of the screen and size it to 75% of the work area
    • Tile windows along the top, bottom, left or right side
    • Maximize windows so that they span all monitors
  • Integrated Flickr image search & download
  • Drag maximized windows by their title bars to other screens