xdesktopwaves puts a nice rippling pond effect on your X desktop.

xcompmgr (in the experimental Debian repo) adds true transparency to
your desktop.

I've played with both, and they are fun (and pretty easy to set up if
your video driver supports the composite manager). On today's machines,
the overhead isn't noticeable. Of course, you can always opt for
compiz/beryl if you want the full 3-D experience.

In the end, though, I just drop back to pseudo-transparency. Anyway, if
you haven't tried these little applets out, do so!

Tux rox!