I was just browsing through some of the other form-related scripts on javscript source. I'm sure I've linked to this in the past, but these types of widgets come up a lot in the work I do. I usually just avoid them, but it would probably be a lot easier for people if I actually used the javascript to "simulate" how the application I'm designing for will work. So it's worthy of another link (at least to me)
Javascript Forms: Menu Swapper
Move selected items from one list box to another list box.