Auto Europe - The consensus of opinion that I have heard is that Auto Europe usually has the best rates and are the easiest to deal with. Also if you book through this link you will be making a small donation to the Slow Travel web site that is dedicated to Travel, especially in Italy.

Europcar - Online booking and a location at the Rome and Florence airports.

Hertz - A US based company with International offices. Online booking and a location at the Rome and Florence airports. Hertz offers "Club" deals that might be more competitive.

Tiger Car Rental - provides low cost car hire in thousands of locations throughout Europe and Worldwide.

Car Rentals - This link is to the CarRentals.co.uk "Rent a Car in Italy" page.