I took the Ciao Roma open top Bus tour. The closest bus stop to the hotel was stop #12 next to Carrani Tours office. Via Vittorio Emanuaele Orlando 95 (Tel 064742501.) where I bought my all day pass for €19.
Buses pass every 30 minutes throughout the day, and you hop off and hop on wherever you please at all the major sites.
The bus supplies headphones with a tour commentary in several languages.
Howard [ Just Imagine ] 16-Sep-2007