A bronze Judith & Holofernes by Donatello. The original sculpture is inside the Palazzo Vecchio Museum (and is much more arresting - see the last two photos) - but you can see the replica in the first images below outside the Palazzo Vecchio. The text panel (also from inside the museum) has a good history of the sculpture, which was originally in the Medici Palace on via Cavour until it was taken by rampaging Florentines when they turned the Medici out of Florence. It was brought to the Piazza in 1495 and placed in various locations including the Loggia dei Lanzi over the centuries until ending up in this spot.