Rental car

I used a consolidator (www.autoeurope.com) but I also used Kayak.com. The major multinational companies are present there in Florence: Avis-Budget, Hertz, Europcar, Thrifty, National-Alamo, etc. Europcar has the most comprehensive network in Italy and they tend to have slightly better prices in my experience. All rental companies will try to scam you, but that's what they do. Just make sure you buy full comprehensive no deductible insurance (insurance is mandatory in Italy and your insurance or credit card insurance won't work). Also make sure you keep the receipt of your gasoline and show it to them. Take a look at the charges. Inspect the car well and take pictures of dents and scratches if any.
Pick up the car in Florence anyplace. All downtown locations for all companies are on via Borgognissanti at the intersection with via Maso Finiguerra. Hertz has a location also on via del Sansovino. Europcar has one at Novoli. You need a taxi to get to those. You can also rent at the airport, but the daily/weekly rental rate will be significantly higher.
Exiting the ZTL you will not incur any fines, so driving out of the city is not a problem. Make sure to have accommodations outside the ZTL (or better outside the city) while you have a car.

We also used AutoEurope on 2 different occasions. They are located in Maine and it was easy to deal with them both before the trip when securing my reservation and during the trip when we had a small hiccup. We stayed in Siena and therefore rented the car in Siena so I can't comment on renting in Florence but I would follow Roberto's advice on that. Be sure to get an Intl Driver License from AAA before you leave the U.S. It is mandatory in Italy (though we've never been asked to see it). Oh, and be sure to take a portable GPS with you (preferably one that you've used and gotten used to prior to the trip). We have successfully used our trusty Garmin GPS both times we drove in Italy and would not dream of driving there without it. Have a great trip.

Roberto's right on the money with his advice.

We went thru AutoEurope.com and picked the car up at Hertz, 53 Via del Sansovino. The gave me directions to get out of town.

Since we were going to Rome, we turned the car in at Hertz in Orvieto and took the train into Rome Termini.
I've got a Hertz corporate card that gets a big discount. If you join their frequent renter club, sometimes their direct rates are discounted or you can get a 1 class free upgrade. Just compare all inclusive rates.
We found Siena difficult to deal with, as the city is very congested with apartment complexes, etc. Parking's difficult. I'd prefer to go there as a day trip out of Florence via bus.
We stayed on an agriturismo outside of San Gimignano/Certaldo and visited hill towns, including Volterra, I never got around to getting any road maps. We found the road signs to be well marked, the pavement good and that Tuscany/Chianti is pretty easy to navigate in.

Have a great time.

Do not confuse autoeurope.com (good) with autoeuropa.com (bad). Two different companies.

I prefer to deal directly with a company and not a third party. For our upcoming trip to Italy I made my car rental reservation directly with Hertz (I have a good discount through my insurance company). Later I got a 1 week sale advertisement from AutoEurope for a rental for the days and locations I was using; it was a Hertz car; and it was about 10% less than my quote from Hertz. I just called Hertz and they matched it.

The same goes for hotel reservations. If I find a rate on Booking.com, for example, that is less than the hotel's website rate I just email them and they have, so far, always matched the rate.

If there is any problem, cancelation, etc. I would much rather deal direct than through a 3rd party. Yes, check the third party rates, but then go to the primary vendor and deal directly.

We also rented from AutoEurope. The actual car rental agency was Europcar and we picked up the car on Via del Gelsomino, on the south outskirts of Florence. From there it was easy to avoid city traffic and the dreaded ZTLs, and head south. AutoEurope is easy to deal with; you can give them a telephone call and get all your questions answered. Have a great trip.