Usually steel, sometimes aluminum (such as the Audi A8) or fiberglass-epoxy composite (like the Chevy Corvette). You might find some body pieces made with carbon fiber-epoxy composites (the Lamborghini Gallardo Superleggera uses a carbon fiber spoiler, mirrors, and underbody panels), but mostly, carbon fiber is an aftermarket material, because of the cost and production time.
Steel is cheap, strong, and absorbs a lot of energy in a crash, which keeps you safe; it's easy to form, easy to repair, easy to recycle, and there's quite a large investment in it already. It also lasts about as long as the rest of your car. So most car manufacturers are sticking with it for now. But if fuel economy becomes more of a concern, light weight will become more important, and you might see more companies moving to other materials.
Hope that helps!