Basilica of San Pietro in Vincoli

Its name — 'Saint Peter in Chains' — refers to the relic displayed beneath the high altar, said to be the shackles that bound the apostle in Jerusalem and Rome. The bigger draw for most visitors is Michelangelo's sculpture of Moses, originally intended for the unfinished tomb of Pope Julius II, and considered one of his most powerful works outside the Sistine Chapel. Because it sits a short but uphill walk from the Colosseum rather than directly on the main tourist path, it gets a fraction of the crowds despite the caliber of what's inside. It's a quick, worthwhile detour for anyone touring ancient Rome who also wants a dose of Michelangelo.