Gnome Screen Ruler is an on-screen ruler for measuring horizontal and
vertical distances in any application.  It has the following features:

    * Horizontal and vertical display
    * Multiple units: pixels, inches, centimeters, picas, points,
      percentage
    * Configurable colors and font
    * Can be set always-on-top
    * Measurement lines track mouse cursor to help measure
      anything on screen
    * Move and resize via click-and-drag with the mouse
    * Align with other windows while dragging (hold Shift key)
    * Move using the keyboard for precision (arrow keys)
    * Resize using keyboard (Alt+arrow keys)
    * Speed up keyboard operations (hold Shift key)

WWW: http://gnomecoder.wordpress.com/screenruler/