The Plugin2 allows a user to detach applets from webpages. Also, it provides some API to let applets make their detached window translucent. This works fine on MS Windows. However, on X11 this requires one additional step: the applet's background must be set to a transparent color:

applet.setBackground(new Color(0, 0, 0, 0));

See a corresponding bug in an AWT test that's been fixed this way: 7008513.