PBS
Pbs Great Performances: Porgy and Bess an American Voice
Revisit the original production of "Porgy and Bess" with interviews, photos, and stories never told. Although the site doesn't have any clips from the musical, it does provide links where you can find them.
Other
Opera Houses and Companies on the Web: 5 Continents
This is list of links to a selection of websites of opera companies. Not all-inclusive, it is large and a good place to begin looking for places that perform opera all over the world.
Other
The Metropolitan Opera
The Metropolitan Opera, or "The Met," certainly one of the best known American opera companies, if not the #1 American opera house.