A large amount of links plus information on how to search the Web to find information. Despite this, they are much more comprehensive than the CIA World Factbook, especially with regards to history. Fine Arts Aria Database Search for arias by various types of criteria. Includes character, opera, language, synopsis, and other interesting info. Classical Insites Can be very difficult to get through, with a frames browser and little definition.
Biographies are also weak in terms of people covered.
Aside from that, there is a good glossary and reasonable coverage of composers. More of a ballet page than anything else, this page has a large number of links. Check out the Ballet FAQ for a general introduction to ballet, or peruse the list of ballets to know more than you ever wanted about a specific one. I'm including this page on the assumption that more will be added in the future. As of right now, there is a comprehensive list of musicals with song titles.