opacity:
xOffset:
yOffset:
xSize:
ySize:
borderSize:

To use:

Copy these four images to your site:

Copy this javascript file to your site: shadower.js
Adjust the settings at the top to your liking. Most importantly, the location of the images.

Add a reference to the script in your page:
<script type="text/javascript" language="javascript" src="shadower.js"></script>

Load the shadows. Doing it after everything is finished is probably good:
<body onload="s=new Shadower();s.install();s.update();">