The Society of St. Romanos the Melodist Society promoting and publishing Liturgical Chant in English. The Society publishes fine books like those available here.

PSALM The Pan-Orthodox Society for the Advancement of Liturgical Music

The Divine Music Project Lots of Byzantine Chant from St. Anthony's Monastery. Used the be called "The Divine Liturgies Music Project."

Greek Liturgical Texts Complete Liturgical Texts in monotonic and polytonic Greek. An excellent resource compiled by Fr. Leo Scheffe and hosted by TCG Alaska

Liturghia  A complete Divine Liturgy in Romanian Chant (and in the Romanian language) by Catalin Francu using Lilypond. 

St. Nicholas' Choir Corner" Some scores in English from St. Nicholas Church in Asheville, NC

Anthology of Serbian Chant Serbian composer Nikola Resanovic has posted a very complete Sbornik of Serbian Chant (in English).

Kurt Sanders Orthodox Choral Music Website Many scores by Kurt Sander and other interesting materials regarding Orthodox Liturgical Music (and Arvo Pärt).

Traditional Polyphony of Georgia A fascinating and extensive site on Georgian music, including an informative section on the history and meaning of Georgian Sacred Polyphony.