[Xmonad] compositing

[John S. Yates, Jr.]

Not too long ago this screen shot used to appear on the xmonad homepage:

   http://xmonad.org/images/xmonad_xcompmgr.png

It was on the strength of the image that I started to play with xmonad.  I now have xmonad working and have even enabled a few extensions.  But for
the life of me I cannot find any description of how to achieve the shadow effect.  Can anyone help?

TIA,

/john