There are literally hundreds of street festivals of all kinds each year in Italy. I personally recommend Terzo Studio for anyone looking to play in Italy. Their big festival is "Mercantia" at Certaldo (near Siena), but they organize many festivals all over Italy. They also publish a fine catalogue of street artists in Italy every year with photos by Massimo Agus. You can find them at...www.terzostudio.it or the Mercantia site... www.mercantiacertaldo.com
A.M.I. (Association of Itinerant Musicians) has one big difference from other organizing groups, it was founded by, and is completely made of street musicians and fighters for the cause of street music. It's founder, Claudio Montuori has been for many years involved in changing the laws in Italy against street performing. The A.M.I. have also become connected with organizing festivals for various communities in the Rome area.
An example of a festival that's way out of hand is the Ferrara Busker's Festival. I strongly recommend staying away from it for artists and public alike. The village is incredibly over crowded, and the police are well known for using extreme force when none at all is necessary. But, if you absolutely must see it, go to...
www.ferrarabuskers.com ...just to see how they've managed to make busking more commercial than Disneyland!