The Torre dell'Orologio is the blue-and-gold astronomical clock above the archway on the north side of Piazza San Marco. It is easy to admire from the square, but the better visit is the guided tour through the mechanism and up to the rooftop bells.
The clock was restored after years of closure and remains one of Venice's most distinctive pieces of civic showmanship. Book ahead if you want the inside tour; access is limited and usually tied to fixed language times.