PDR Editions

Sintra International Singing & Choral Conducting Course

Sine nomine Seattle

Bellevue Chamber Chorus

Northwest Chamber Chorus

Choral Public Domain Library

Another great source for public domain scores is the International Music Score Library Project (IMSLP).

Alain Naigeon has a great web site for Renaissance music with a very good explanation of ligatures and mensural notation.

For articles on temperament visit the temperament page at AudioMastermind

AudioMastermind also has some great articles on mean tone and pythagorean tunings.