Auto City
Hire cars and Smart cars for rental.
http://www.auto-city.it/

Auto Europa
http://www.sbc.it/

Avis
http://www.avis.com/

Dollaroexpress
http://www.dollaroexpress.it/

Europcar
http://www.europcar.com/

Free Rome
Explore the historical centre of the city in an electric vehicle.
http://www.freerome.it/

Happy Rent
http://www.happyrent.com/

In Italy
An excellent primer on hiring a car and driving in Italy.
http://www.initaly.com/travel/lomax.htm

Maggiore Budget
http://www.maggiore.it/

Rome Limousine Service
Just as it says.
http://www.romelimousineservice.com/

Thrifty
http://www.thrifty.com/