Boxol.it
Book tickets to shows and events in Rome at the click of a mouse.
http://www.boxol.it/

Oikos.org
Index of ticket offices in the capital.
http://www.oikos.org/hosticket.htm

Select Italy
Search for and book tickets to musical and cultural events nationwide.
http://selectitaly.com/

Weekend a Roma
Book tickets to Rome museums, galleries and tours with this agency.
http://www.weekendaroma.com/