Appian Line
Well known bus tour agency.
http://www.appianline.it/

Carrani Tours
Find out more about Carrani's bus tours of Rome at its website.
http://www.carrani.com/gb/viaggi/metaeindex.htm

City Sightseeing
Hop-on, hop-off tours of Rome by open-top double-decker bus.
http://www.city-sightseeing.com/
http://www.guidefriday.com/

Green Line Tours
Full details of the firm's bus tours can be found here.
http://www.greenlinetours.com/

Italbus
A coach company with a range of tour services for visitors.
http://www.italbus.com/

Odyssey Tours
Operates bus tours in Rome and further afield.
http://www.odyssey-tours.com/

Stop 'n' Go City Tours
Bus tours of Rome that depart hourly.
http://www.romecitytours.com/

Vastours
Offers a range of bus tours of Rome. The website is in Italian only.
http://www.vastours.it/