Choral Public Domain Library
http://www.cpdl.org/wiki/index.php/Main_Page

Werner Icking Music Archive
http://icking-music-archive.org/

Acadia Early Music Archive
http://ace.acadiau.ca/score/archive/ftp.htm

MusiXteX (scores of interest are by Dunstable, Tallis, Byrd)
http://www.mab.jpn.org/musictex/index_en.html#scores
http://www.mab.jpn.org/musictex/chorus_idx_e.html

Laymusic
http://www.laymusic.org/music-publish.html

Mutopia
http://www.mutopiaproject.org/cgibin...strument=Voice

Cipoo.net
http://www.cipoo.net/music_a.html

Gallery Music
http://www.psalmody.co.uk/contents/music.html