Some of these sites straddle the line between free and pay or between English-language or not. We've given it our best shot to compartmentalize them, with the caviat that this might not be totally accurate.
OCA Dept. of Liturgical Music - offers troparia and kontakia for forthcoming Sundays and keeps them on the site for archival purposes (they started Jan. 2002). The program is managed by Walter Obleschuck and Paul Meyendorff, PhD. While each hymn is offered in Russian plain chant, most are offered in multiple settings, including Galician, Bulgarian, Carpatho-Rusyn and many others. Tutorials for different chants are offered here. They also offer a huge collection of Paschal troparia settings.
OCA Diocese of the West, Department of Liturgical Music - maintained by Fr. Lawrence Margitich, offers a variety of music from different musical backgrounds, including Slavonic and Georgian languages.
St. Gregory Palamas Monastery, Ohio - Hieromonk Seraphim Hagiopavlitis has translated a number of hymns into English, in a meter that matches the original Greek; this allows the hymns to be sung to their exact Byzantine melodies. He has also set a number of hymns to Western and Byzantine notation; some of these are available on the site, while others must be purchased.
St. John the Theologian Choir Page - This Antiochian choir director makes quite a bit of music, from many different traditions, available on his site.
Arimathea - Vast amounts of Byzantine chant in Western notation are available for free download here.
Evening Song - This RUSSIAN LANGUAGE SITE offers a number of predominantly Znamenny Chants (some Valaam, too), mostly scanned from old service books. There are a number of texts from books about chant, both theological and practical in nature. To download chants from this page, you need to be able to open RAR compression files (a variant of ZIP compression); this software can be found in a freedownload from the web.
Moscow Choir Directors Seminary - This RUSSIAN LANGUAGE LINK goes to a page for downloading pages from the 1909 Synod Obikhod and the 1899 Synod Triodion. It contains hundreds of pages of Slavonic-language chant for Liturgy and Vigil in Znamenny, Gretchesky, Kieven and Bulgarsky chants. All notation is square-note.
Synod Cathedral Choir of New York City (ROCOR) - Mostly Russian plain chant, with some choral settings, all settings are in, at least SATB. This site offers a HUGE collection of troparia and kontakia, possibly complete for the year, in the Russian usage. There are many other settings here, as well.
Znamenny Chant - This RUSSIAN LANGUAGE SITE contains numerous settings in Slavonic, Znamenny, monophonic chant.
Kliros - This RUSSIAN LANGUAGE SITE contains settings from a variety of backgrounds in Slavonic.
Valaam Chant - This RUSSIAN LANGUAGE SITE makes a number of items from the Valaam Obikhod of 1912 available in Slavonic; many of these hymns are offered in English by St. Raphael Press.
Musica Russica - Vlad Morosan, a giant in Russian Orthodox music in the United States, makes very comprehensive offerings on his site - CDs, sheet music, books and more. Sheet music is available in both English and Slavonic; recordings come from all across the board and include quite a bit of non-liturgical recordings, too.
Psalm, Inc. - This pan-Orthodox organization offers a wide variety of music, articles on music, workshops and networking opportunities. They manage the e-mail list "Psalmnotes" and offer a biannual newsletter
Antiochian Archdiocese Department of Liturgical Music - site offers a number of free downloads of music, but you can also find information about purchasing their 3000-page Byzantine Chant Project, the only one of its kind in English.
St. Vladimir Seminary Press - The Seminary bookstore offers a number of collections of music, covering chants from many different backgrounds.
St. Tikhon's Orthodox Theological Seminary Bookstore - This bookstore offers a number of books for sale, both service books (text only) and liturgical music books, including Fr. Soroka's "red," "green," "brown," and "blue" books. Be sure to check out their CD/tape of the glorification liturgy for St. Raphael of Brooklyn (our patron). You might also be able to call them and get a mounted print of St. Raphael's icon.