Crude (as in "unrefined", not "spits on the carpet") iron, popped fresh from the blast furnace. How'd it get that goofy name? 'Cause the iron removed from the smelting furnace is very vaguely shaped like a pig -- it's kinda largish and oblong. The molds they pour the unrefined iron into are also called pigs. I guess when you work in a steel plant, you gotta work your imagination somehow, right?
UPDATE: Uberbanana says: "I was recently watching an industrial construction show on either the History or Discovery channel, and they explained the reason why it is called pig iron is because the way the troughs and molds that the molten iron is poured into are laid out, it looks like several piglets suckling from their mother. After the metal has cooled and each individual bar is removed, each piece is called a pig."