Although selling music is a primary purpose of this site, it has useful descriptions about many kinds of music and musical genres, and short biographies of famous musicians. A blog and reviews are also important features on the site.
Describes the special collections of American music at the Library of Congress in Washington, DC. Useful as a summary of the kinds of music Americans have produced, especially popular song and musical theater.
In addition to finding aids, this site features special exhibitions and permanent presentations of digitized selections from the collection. Parts of the site include recorded music in RealAudio or MP3 format.
Though such lists are always open to quibbles, this one serves as a provocative and stimulating review of a century of the best music produced in the United States. The music can be heard by Real Audio links.