Three of them, in fact. The original in Galleria della Accademia; the (marble? plaster?) copy in front of Palazzo Vecchio in Piazza della Signoria; and the (bronze?) copy in Piazzale Michelangelo.

I learned this the hard way when I was going to meet a friend there. Neither of us were locals, so we wanted to choose an unambiguous place to meet. What could be more iconic than "let's meet by the David"? Hilarity ensued.