Glass!
Right now, using any glass effects (under Vista Aero) is really, really painful. Unless you ask, I won't go into detail, as it's a fairly obvious request, but there's currently a bit of a vacuum in that market.
I see it as a class that inherits from Form, much like RibbonForm does. If you can 'simulate' the glass effect in some limited manner under XP, all the better.