http://www.metguild.org
2. Metropolitan Opera
http://www.metopera.org
3. Operissimo
http://www.operissimo.com
4. Opera America
http://www.operaam.org
5. Opera News Online
http://www.operanews.com
6. Operabase
http://www.operabase.com
7. San Francisco Opera
http://www.sfopera.com
8. New York City Opera
http://www.nycopera.com
9. OperaWeb
http://www.opera.it
10. The Aria Database
http://www.aria-database.com