Other
Music History Online: Music of the 20th Century
This page covers a wide range of topics concerning 20th century music. Included are many references to styles and composers and terms found in 20th century music.
Other
Oculus Magazine: Early Jazz
Written for an audience accustomed to rock, this is a good history of Jazz, from the late 1800's, when the form supposedly took shape, to recommended recordings and books.
Library of Congress
Loc: America's Story: See, Hear and Sing
Lots of history of American music with fun links and audio clips, most from the 20th century. A neat site to learn more about America's past.