For a list of other music newsgroups, this site lists both scholarly, and general interest groups. It is extremely informative and comprehensive. The Mammoth Music Meta-List is, as its name suggests, a massive site which provides information on all types of music, including classical and opera. It also lists archives, music services, and other collections of music resources. This bibliography of music reference tools lists several music resources.
Brief annotations are available for each source.
This site provides brief information on composer biographies and selected works. Bach, Beethoven, Brahms, Chopin, Elgar, Handel, Haydn, Holst, Mendelsohn, Mozart, Scarlatti, Schubert, Schumann, Shostakovich, Sibelius, and Vivaldi. There is lots and lots of room for expansion for both biographical and selected works information, but it's a good start. This composer site is actually within the more general classical music information source. It's extremely informative and comprehensive.