If you prefer a public parking structure, we offer some available in central Rome as of 2010. All charge fees, of course, but we haven't included them because they are subject to change.
Terminal Gianicolo (not too far from St. Peter's)
Open 7 am - 1:30 am
Entrance in Piazza della Rovere/Via Gregorio VII
This may be the most convenient structure, because there is an electric bus, #116, which has its terminal right inside the structure and goes every seven minutes to Via Giulia, Campo dei Fiori, Corso Vittorio Emanuele, the Pantheon, Via del Tritone and P.zza Barberini. If you plan to avail yourself of this very simple system, be sure to stop at a tobacco shop or newsstand before you enter the car park, to purchase bus tickets.
Stazione San Pietro (also near St. Peter's)
Open 6 am - 10 pm, Monday-Saturday
Entrance at St. Peter's Rail Station
Cheaper than the Gianicolo (2 Euro for 12 consecutive hours), right next to the Metro A subway, which goes to the Vatican and the Spanish Steps.
Sorry, no contact information available!
Parcheggio Ludovisi (near the Spanish Steps)
Open 5:30 am - 1:30 am
Entrance at Via Ludovisi 60, a quick walk from P.zza di Spagna or Via del Tritone
Costs from 2.20 Euro/hour to 18 Euro/day
Villa Borghese (near Via Veneto)
Open all day
Entrance on P.zza Brasile
Bus #116 or #95 to Via del Corso and P.zza Venezia, or use the underpass to the Metro A subway, which goes to the Spanish Steps and St. Peter's