The Tempietto (which literally just means "little temple") is located on the grounds of San Pietro, a church in the Monteverde neighborhood, in Montorio (close to Trastevere and the Giancolo). There are several spots in Rome that supposedly mark the location of St. Peter's crucifixion, and this is one of them. The Tempietto was designed by Bramante in the early 1500s, and while it is incredibly small, the building is one of the most cohesive, textbook examples of Renaissance architecture.
The hours when you can get in to see the Tempietto are few, but you can always view it through the iron gate that leads into the courtyard of San Pietro in Montorio. If you're lucky enough to be there during the open hours, you can walk around the back of the circular building and view a tiny altar marking the location of the first pope's martyrdom. You can't actually enter into the Tempietto itself (it's so small that there wouldn't be much point anyway), but it is incredible to see what Bramante did with such limited and confined space. The view of Rome offered by the small piazza in front of the church is an added bonus.