PDR Editions

Sintra International Singing & Choral Conducting Course

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

Bellevue Chamber Chorus

Northwest Chamber Chorus

Choral Public Domain Library

If you're interested in alternatives to equal temperament you should look at Scala.

For articles on temperament visit the temperament page at AudioMastermind

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