Eras Online - Basics about the major periods of classical (art) music, with terms, composers, RAM audio examples, and purchase links. From Essentials of Music, sponsored by Sony Classical and W. W. Norton.
Gallery Music - (Sung by church and chapel "bands" in the UK in the 18th and 19th centuries) Includes articles, sample scores and MIDI files, links, and organizations.
History in Song - Includes annotated texts about American history, from Revolutionary War songs to 1960's anti-war protests.
The History of Czech Music - A brief history of Czech music, from the Middle Ages to the 20th century; Biographies and work of important composers. (Radio Hudba, Czech Republic)
Jewry in Music - Articles and links about the entry of Jews into the musical professions in Europe in the early nineteenth century.
Major Musical Periods - Graphic timeline: typical musical forms and styles, major composers, sample MIDI files
Medieval Music and Arts Foundation - Early Western and traditional non-Western music: Recording suggestions, discussion, articlesl links.
Music and Chess - An annotated bibliography of musical compositions related to chess.
Music History 102 - (at Internet Public Library) Very brief overview of major periods, with 20+ representative samples (RealAudio)
The Music of Freemasonry - Masonic influences on Classical music, and music in the Lodge: Notes, MIDI samples, and facsimile pages.
The Musik Makers - History of English speaking popular music, from 1500 AD to the present day. Includes timelines, styles, artist profiles, and the Hal Shaper collection of 20th century songs.
Save Our Sounds - Describes a project by the Smithsonian Institution and the Library of Congress to convert important collections of historical recordings to modern formats.