Composers are generally people who write music. A strong distinction is made, however, between composers, lyricists, and performers in such genres as classical and jazz music. In other genres, such as popular and folk music, the term "songwriter" means someone who authors both music and lyrics, and is more common than "composer." If a songwriter also performs, he or she is called a "singer/songwriter."
This page includes a list of composers sorted by various genres.
OperaGlass: Opera Composers - Alphabetical listing with works and links to better-known composers with details of certain compositions. From Stanford University.
U.S. Opera - Information about American opera composers and their works.
Mario Paint Composer - Mega Man 9 - Concrete Jungle