Rome: Hadrian's Temple
Hadrian's Temple is a temple to the deified Emperor, Hadrian, on the Campus Martius in Rome, built by his adoptive son and successor Antoninus Pius in 145 and now incorporated into a later building in the Piazza di Pietra. Borsa Bank now occupies the space.
At first I thought this was the Pantheon. What did I know? About an hour later we stumbled across the actual Pantheon and deemed Hadrian's Temple Pantheon West, so we wouldn't confuse it with the other Pantheon.
Note to self: in the future, no Chianti before sightseeing.