I have used Magic Sculpt to put a zombie face onto a mannequin, to make a model of Dracula’s castle for the movie Van Helsing, and to make small sculptures and other props for Star Wars. Magic Sculpt is a two-part epoxy putty with the consistency of clay. You have about an hour to work with it and then it hardens like a rock. Carves beautifully and smooths with some water. It is better than Fimo, Super Sculpey, and other polymer clays that need to be kept small so the pieces are heated evenly when you cure them in an oven. This stuff cures chemically (not with heat or air) which means you can make pieces as large as you want — as big as a life-sized statue. It does not crack or shrink. You can also add more Magic Sculpt to a already hard piece, which you can not do with polymer clay, building a complicated sculpture in pieces. After it cures you can drill it, sand it, and paint it.
[I've found Magic Sculp and Magic Sculpt online. They appear to be the same thing. If you know the difference, please tell us about it in the comments. -- Mark Frauenfelder]