We checked out the Roman Pantheon ('temple of all gods'), which turns out to be a big, round room with a dome and a foyer. It's really well preserved and pretty darn cool to see. It also has a large obelisk out front.
Fun fact: the diameter of the room and the height of the dome (oculus) are both 43m (142 ft).
The Pantheon was originally constructed during the reign of Augustus (27 BC - AD 14), but burned down and was ordered rebuilt by Hadrian around 126 AD. In year 609, the Catholics turned it into a church dedicated to St. Mary and the Martyrs (sounds like a band name). I'll let the pics speak for themselves.