Small plugin that reports the size of the browser's scrollbars in pixels.

jquery.scrollbarWidth.js

/*!
 * jQuery Scrollbar Width v1.0
 * 
 * Copyright 2011, Rasmus Schultz
 * Licensed under LGPL v3.0
 * http://www.gnu.org/licenses/lgpl-3.0.txt
 */

(function($){

$.scrollbarWidth = function() {
  if (!$._scrollbarWidth) {
     var $body = $('body');
    var w = $body.css('overflow', 'hidden').width();
    $body.css('overflow','scroll');
    w -= $body.width();
    if (!w) w=$body.width()-$body[0].clientWidth; // IE in standards mode
    $body.css('overflow','');
    $._scrollbarWidth = w;
  }
  return $._scrollbarWidth;
};

})(jQuery);

Great stuff, seems to work well and so simple. :-)