Simple jQuery plugin that checks if an element is in the visible viewport

visible_plugin.js

(function ($) {

  if ( typeof $ !== 'function' ) {
    return false;
  }

  $.fn.inViewport = function () {

    // assign this object to local variable.
    // use raw DOM node, not jQuery objects.
    var el = this[0];

    // get element offsets
    var top = el.offsetTop,
        left = el.offsetLeft,
        width = el.offsetWidth,
        height = el.offsetHeight;

    while ( el.offsetParent ) {
      el = el.offsetParent;
      top += el.offsetTop;
      left += el.offsetLeft;
    }

    return (
      top >= window.pageYOffset &&
      left >= window.pageXOffset &&
      (top + height) <= (window.pageYOffset + window.innerHeight) &&
      (left + width) <= (window.pageXOffset + window.innerWidth)
    );
  };

}(jQuery));

Nice!