Recently both my brother in Australia and another person I have heard of in the UK have received fines for driving into a ZTL. This is something to be really careful about as most visitors from other countries would not know what a ZTL is.
A ZTL is a Zona a Traffico Limitato. Many cities and towns in Italy have such areas. The may be camera controlled and they may be active at all times or only at specific times. Only authorised vehicles may enter a ZTL and if you get your photo taken, even with a hire car, the fine will follow you home. What is worse is if your road takes you in and out then back in to a ZTL you might finish up with two fines within minutes of each other.
This sign indicates that the ZTL is active on work days (crossed hammers) from 7:30 to 17:30. If you have booked a hotel and/or parking within the ZTL you must make sure that the hotel or parking station notifies the local police the numberplate of the car.
This is another one in Firenze, but at least it is in two languages and also has a traffic light.
The crossed hammers symbol is also often used on parking signs as well. Regarding parking: if it is not a paid parking zone (blue lines on the road) or a residents only parking (yellow lines) you will probly have to display the time you arrived using the hour disk. Free parking, whether for a limited time or not, is normally indicated with white road markings.