San Luigi dei Francesi sits between Piazza Navona and the Pantheon and would be worth a look even without its famous chapel. The reason to come, though, is Caravaggio: three paintings on the life of St Matthew hang together in the Contarelli Chapel.
The church is central enough to fold into any walk through this part of Rome, but give the chapel a few quiet minutes. The paintings are among the city's great free art experiences.