[Xmonad] compositing

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