Renting a car in France is similar to renting a car elsewhere, especially since the market is dominated by the familiar international companies (e.g. Budget, Hertz, Avis). However, there are two areas that one needs to be carefull about: Insurance and Mileage.
France Car Insurance
The first thing to be aware of with car insurance in France is that in the event of an accident, you are required to complete a special car accident form, detailing exactly what happened, with both your signature and that of any other car divers involved in the accident. If this form is not completed and turned in to your insurance company within 24 hours, you may find yourself uninsured. It is a strange fact of French law that failure to meet this deadline may leave you uninsured and responsible for damage, even if the other party is at fault and admits it. One can see a true life example of this at France Car Accident.
Consequently, when renting a car, one should ensure that a copy of the form is in the car (it is usually in the glove compartment), if possible ensure that you have two copies (more than one copy is required if there are more than two cars involved in an accident). If you don't understand French well enough to read the form, you should also have an English translation of the form to ensure that you do not tick any boxes incorrectly (a tick in the wrong place can easily result in you accepting blame for the accident!!). An explantion of the form and an english translation can be found at France Car Accident Form.
Aside from this specific issue, some general information about insuring your car (be it rental or owned) is provided in France Car Insurance.
When receiving or comparing quotes for rental cars, one needs to look carefully at what is quoted. This includes not only the type of car, but also the amount of insurance. In particular, the amount of deductible; a quote for a rental car with a 100 euros deductible is naturally going to be more expensive than one with a 1000 euro deductible.
Car Rental Mileage
When comparing quotes, another item to look at carefully is how much mileage is included. If you are planning to drive a lot in France, it is normally best to get a package which includes sufficient mileage, as some rental firms charge very high mileage rates if you exceed the amount included in the rental package.
The article Driving in France provides additional information on rules and signs.