Il Ponte dei Sospiri (Bridge of Sighs in English) is one of the most famous landmarks of Venice. It connects the Doge's Palace to the New Prison. It was originally built for prisoners crossing to the prison, it'd be their last glimpse of sunlight.
It's a picturesque spot in Venice and a very popular one, so expect it to be always crowded. If you're like me and love to see popular sights with fewer crowds, then going early in the morning is the best thing to do.
By the way, this view of the bridge is from Ponte della Paglia.