Robert H. Sarkissian
Island of Freedom: Guillaume De Machaut
This site contains info on the life of Machaut and some of his famous works. Information is from the Grolier Multimedia Encyclopedia.
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.
Classical Net
Classical Net: Johannes Ciconia (1335 1411)
A brief paragraph outlining the life and work of Medieval composer Johannes Ciconia.