A program for measuring the pixel size of on-screen elements. Intended for
GUI debugging to verify widget separations and sizes, though may also be
used for general computer graphics debugging purposes, including measuring
pixel distances of images, web applications, etc.

You can easily make copies of the ruler and reoriented it via either the
keyboard or mouse.

WWW: http://seriss.com/people/erco/fltk/flruler/