The Gates of Paradise, made by Lorenzo Ghiberti from 1424 to 1452. These are actually copies,
the originals being moved in 1990 to the Museo dell’Opera del Duomo. There is another copy
made in the 1940s in the Grace Cathedral in San Francisco.
In the tourist season, the doors are generally thronged by hoards of people, and it's very
difficult to get a clear photograph of them. After much patience, I was lucky to get one,
which is in my "Doors" gallery, here: http://www.pbase.com/chrisg_62/image/139341213