[tstegart]

Clay, paper mache, foam, legos, wire, sheet metal, balsa, glass, wood. You can carve a mold and inject molten plastic (styrofoam beads) at home (preferably well ventilated). You can also melt soft metals and use sand molds.

Thats simple prototyping. To get an exact replica, in the same materials you want to use for final production, there are shops that will make plastic or metal molds from CAD files, or can carve with lathes, etc.

Edit, I should add wax. Its actually an incredible material for prototyping. It carves especially well, and you can add pieces if they fall off accidentally, or you decide to change the design. If you don't like it, melt it down and form a new block. Lastly, when you're ready to make a mold, you can encase it in sand or another substance, and then melt the wax away to leave your shape.

[tstegart]

Like someone mentioned below, resin works as well. its a great way to do additive design, adding material over time to get the thickness or curvature over time.