Sistine Chapel
The Sistine Chapel is primarily used as the Pope's own chapel, although it is also used by the Sacred College of Cardinals when they meet to elect a new pope.
While the outside of the building is somewhat plain and unadorned, the inside is decorated on the walls and ceiling by some really amazing frescoes that were painted by Renaissance masters. Especially noteworthy is the ceiling and front wall that were painted by Michelangelo.