Stanford University
Stanford University: Medieval Music Birth of Polyphony
An article on the beginning of Polyphonic music during the Middle Ages. You'll also find illustrations.
Library of Congress
Loc: The Moldenhauer Archives
The Moldenhauer Archives at the Library of Congress contain approximately 3,500 items documenting the history of Western music from the medieval period through the modern era.
Classical Net
Classical Net: Timeline of Famous Composers: 1100 1700
A PDF timeline of composers in the Gothic, Medieval, Renaissance, and Baroque periods.
Classical Net
Classical Net: Musical Periods
This site contains the most significant artistic output of the most important composers within each historical period.
Wikimedia
Wikipedia: Pierre Attaingnant
A biography of Pierre Attaingnant, one of the first music printers/publishers in the 16th century.
Other
Thirteenth Century Polyphony
Detailed article about polyphonic music and its components in the 13th century.