Other
Lawrence University: A Brief History of Singing
An introduction to the history of Western solo singing from ancient times to the present can be found here.
Other
Diaphragm Description
A post from 'Vocalist' written by Lloyd Hanson, a professor of voice, explaining the diaphragm muscle system.
Other
Thirteenth Century Polyphony
Detailed article about polyphonic music and its components in the 13th century.
Mount Holyoke College
Mt. Holyoke College: Enunciation Exercises
A few exercises to develop enunciation with the singing voice. This site takes you through a five step exercise.