Opera Basic Dictionary  
   Devoted fan Mario Biondi has compiled lists of more than 1,500 operas by more than 500 composers, with their countries and dates, as well as thousands of performers and recordings. Searchable by title, composer, and performer. [English and Italian]
   http://joshua.micronet.it/italian/mariobiondi/opere/diz_opera_.html

About.com: Opera  
   Guide featuring links to selected sites.
   http://classicalmusic.about.com/od/opera/

Operabase  
   News and reviews, performers and artists, opera houses, recordings and web resources, plus booking details, interactive maps, search tools and timelines. [Dutch, English, French, German, Italian, Portuguese, Spanish]
   http://operabase.com/en/

Opera Italiana  
   Links to Italian composers and their operas with biographies, and libretti. [English and Italian]
   http://www.operaitaliana.com/