PNG Drop Shadows
http://www.positioniseverything.net/articles/dropshadows.html

Bergevin, Holly and John Gallant
Position is Everything
2006

Abstract:

This article will describe a modification to existing drop shadow methods, employing semi-transparent .png's. Therefore the method is meant for use in in IE7 and the other modern browsers, but not in IE6 and below, which do not correctly display such .png's.