Josh asked me a while back if it was possible to make a .swf with an alpha (transparency) channel — I said I didn’t think so, but I was wrong, although it’s a feature often being used for evil and not good. Por ejemplo, on this page, there’s an iTunes ad which often appears on the left hand side which actually extends into the text in the center*. In a disconcerting intrusion, at some point the ad actually smashes a guitar and whips a headphone cord over the text; cool, technically, but from a readers’ perspective, godawful horrid.
(link fixed, thanks Trevor!)
*probably using the z-index feature of css, although I haven’t actually checked the code.