One of the stranger museum sites in Rome is Centrale Montemartini, in the post-industrial wasteland (that we like - think of Soho and Chelsea before they became hot) of Ostiense, just outside the city walls near the Pyramid.
Centrale Montemartini was the 20th century thermoelectric plant for the city, opened in 1912, with the 2 large diesel engines installed in 1933. The plant was decommissioned in the 1960s. While the Capitoline Museums on Campidoglio were being remodeled in the late 1990s, the City opened up a new exhibition space in the old plant for much of the Capitoline collection. The result was such a hit, that the City kept the space, even after part of the sculptural collection was returned to Campidoglio in 2005. So Greek and Roman sculptures, new acquisitions by the City, and experimentation in museum display (including how to show the scientific research involved) are the focus of today's museum at Centrale Montemartini.
The spruced up turbines and engines, with classical statuary arranged around them, is a site you simply have to see - hence it rates in our Top 40. It's also Rome the Second Time because the location means many people just don't get there on a first visit to Rome. In fact, we encountered a beleaguered tourist in the Villa Borghese who was asking directions to Centrale Montemartini. He knew it was there, that he wanted to see it, but couldn't afford the time.
Since we, along with Jessica of http://www.romephotoblog.com/, are rather fond of Ostiense's industrial detritus, we included it on Itinerary 4 in Rome the Second Time, right after the Gazometro and just before "Hip Cafes Come to Ostiense."
Open Tuesday-Sunday 9 a.m. - 7 p.m. (early closures Christmas and New Year's Eve), closed Christmas, New Year's, and May 1. Good information is available in English (click on Eng at upper left) at http://www.centralmontemartini.org/. Tickets can be purchased in combination with other City museums.