You'll often see the Palazzo Farnese referenced as the first Renaissance building in Rome. Inside is the French Embassy.
Construction of the Palazzo Farnese began in 1517, commissioned by Alessandro Farnese. Alessandro was made Pope in 1534, at which time he commissioned Michalangelo to finish the third story and give the building some weight befitting the power of the Farnese family. Michalangelo added the deep cornice and added an architrave to support the huge coat of arms you see in the center.
In the 18th century, Palazzo Farnese was considered to be one of the Four Wonders of Rome along with the Colosseum, Castel St. Angelo, and the Pantheon.
This picture shows the view from the street linking the Campo di Fiori to the Piazza Farnese, as you enter into the piazza and the palazzo comes into view.