In some cities there are some special buses that use electrical power from wires over the road with a system similar to that used by trains. They have got normal wheels, but are forced to follow the path due to the electrical power supplying. They used normal Italian plates until the 1950s, even if not in all the provinces (click here to see the picture of a filobus from Venice); then, after a long time without any kind of plate, they have had a special plate since 1984: it's blue, with the code of the province (see table... for this plates Roma has got the code RM in place of the full name) on the left followed by the full-colour company logo and by a serial number of the company in white.
At the present time it's possible to find filobus in the following Italian towns: Ancona (picture 3), Bologna, Cagliari, Cremona, Genova, La Spezia, Milano (picture 1), Modena (picture 4), Napoli (picture 6), Parma, Rimini, Rome (reintroduced in March 2005, with plate numbers from RM 501 to RM 530, picture 2) and San Remo (picture 5).
I must thank Guglielmo Evangelista for pictures number 1 and 3, Roberto Pola for the picture number 4 and Adolfo Arena for the pictures 5 and 6.
I'm adding also this no-plate here as I think it could interest someone. In Rome we have two different kinds of public transportation: buses and they have standard plates and trams, little "trains" on railway on normal streets. These trains use electrical power and don't have any plate, but a serial number painted directly on them as you can see from the picture. Actually also buses have got the same serial number painted, so actually trams miss a plate at all. Likely this happens because trams use the railway, since other electrical vehicles (see filobus) have got a special license plate.