Overlooking Piazza Colonna, the 16th-century Palazzo Chigi has been the official residence of the Presidente del Consiglio, Italy’s prime minister, since 1961. Free guided tours, which last an hour or so and must be booked in advance, lead through the main institutional rooms, including the Sala del Consiglio dei Ministri where the Italian cabinet meets.
For guided tour dates and booking info, check the governo.it website and navigate to 'guided tours' via the Presidency of the Council of Ministers tab on the sidebar.