Just showing off XComposite, since I finally got xcompmgr to where it won't bring down my system. Running Xorg 6.7.99.902 on Gentoo.

The first shot has a transparent Konsole over a Firefox window, with a context menu showing nearby in order to show off the shadows.

The second shot is transparency heaven (or hell, if you don't like it). Two transparent Kopete windows overlapping each other and a transparent XChat, overlapping Firefox.