Operas and Other Concerts in Rome

I was wondering if anyone has a good resource for where I can find information on operas or symphonies or other concerts in Rome. I would really love to attend a concert in an open-air ampitheatre style structure under the Italian stars but any information you have would be great. We are on a budget so would appreciate tips on low-cost (or free!) concerts. We will be in Europe this fall. Thanks!