In Milan there about 1-2 concerts every week, it depends, sometimes it's possibile to have no concerts in a week but usually we have at least 1 concert every week. Once we had more places to play, but in the past few years the mayor of Milan with other politicians decided to close some squats (in Italy the squats are called "centri sociali") and it was a big loss for this city.
Now the most "important" places to play are:
Dauntaun (my favorite place, it's in the basement of C.S. Leoncavallo, the guys who organize the shows here play in some bands in Milan, this "collective" is called Freego)
C.S. Cantiere (they also organize reggae and hip hop shows)
Also there are the booking agencies who organize big shows with foreign bands in clubs, but they do it for the money, and it's not what hc is about, so we can avoid to talk about them :)
Yes there's collaboration between the bands, but not only in Milan, I'd say in the whole country, the scene is pretty small, so a lot of bands are friends and people know each other, and if you need a show in a city that isn't your's you can ask to someone you know and it's not that hard to find a date, it's all a diy thing, you know.
Two or three years ago some guys released a diy book called 97-07, it's about these 10 years of punkhc music in Milan, with pictures and stuff.
Here's some bands:
La Crisi (the singer used to sing in the band Sottopressione)
Death Before Work
La Quiete (they're pretty popular, they've toured australia, usa, japan, they're from the centre of italy, but if i'm not wrong one of them now lives in Milan)
The Infarto, Scheisse!
(they're not from Milan, the're from Bergamo, only 1 hour north of Milan)
To The Embers
The Disgrazia Legend
Abel Is Dying
Al Freddo (this was my band, but we broke up last summer :))